Datasheet

ManualsBrandsTEXAS INSTRUMENTS ManualsData collecting ICsData acquisition IC - AD converter (ADC) External WQFN 16

SPI

AV DD DVDD

GND

AIN0P

AIN1N

AIN0N

AIN1P

PDEN

SAR

ADC

SCLK

SDO

REFGNDREF

CHSEL

MUX

S/H

ADS7945

SPI

AV DD DVDD

GND

AIN0

AIN1GND

AIN0GND

AIN1

PDEN

SAR

ADC

SCLK

SDO

REFGNDREF

CHSEL

MUX

S/H

ADS7946

ADS7945

ADS7946

www.ti.com

SBAS539B –JUNE 2011– REVISED SEPTEMBER 2011

14-Bit, 2 MSPS, Dual-Channel, Differential/Single-Ended,

Ultralow-Power Analog-to-Digital Converters

Check for Samples: ADS7945, ADS7946

FEATURES

DESCRIPTION

The ADS7945/6 are 14-bit, 2 MSPS analog-to-digital

• Sample Rate: 2 MSPS

converters (ADCs), with differential and single-ended

• 14-Bit, Pin-Compatible with 8/10/12-bit

inputs, respectively. The devices operate at a 2

ADS7947/8/9 Family (ADS7946)

MSPS sample rate with a standard 16-clock data

• Outstanding Performance:

frame. The devices feature both outstanding dc

precision and excellent dynamic performance; the

– SNR: 84 dB (ADS7945)

ADS7946 is pin-compatible with the 8/10/12-bit

– No Missing Codes

ADS7947/8/9 devices. The devices include a

– INL: 1.5 LSB (max) (ADS7945)

two-channel input multiplexer and a low-power

successive approximation register (SAR) ADC with

• Low Power:

an inherent sample-and-hold (S/H) input stage.

– 11.6 mW at 2 MSPS Operation

The ADS7945/6 support a wide analog supply range

– Auto Power-Down at Lower Speeds:

that allows the full-scale input range to extend to ±5 V

– 7.2 mW at 500 kSPS

differential or 5 V single-ended. A simple SPI™, with

– 1.4 mW at 100 kSPS

a digital supply that can operate as low as 1.65 V,

allows for easy interfacing to a wide variety of digital

– 0.3 mW at 20 kSPS

controllers. Automatic power-down can be enabled

• Wide Supply Range:

when operating at slower speeds to dramatically

– Analog: 2.7 V to 5.25 V

reduce power consumption.

– Digital: 1.65 V to AVDD

Offered in a tiny 3 mm × 3 mm QFN package, the

• Simple Serial Interface (SPI)

ADS7945/6 are fully specified over the extended

temperature range of –40°C to +125°C and are

• Fully Specified from –40°C to +125°C

suitable for a wide variety of data acquisition

• Tiny Footprint: 3 mm × 3 mm QFN

applications where high performance, low power, and

small package size are key.

APPLICATIONS

• Optical Networking

• Medical Instrumentation

• Battery-Powered Equipment

• Data Acquisition Systems

Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas

Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.

2SPI is a trademark of Motorola.

3All other trademarks are the property of their respective owners.

PRODUCTION DATA information is current as of publication date.

Products conform to specifications per the terms of the Texas

Instruments standard warranty. Production processing does not

necessarily include testing of all parameters.

Summary of content (50 pages)

PAGE 1
ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com 14-Bit, 2 MSPS, Dual-Channel, Differential/Single-Ended, Ultralow-Power Analog-to-Digital Converters Check for Samples: ADS7945, ADS7946 FEATURES DESCRIPTION • • The ADS7945/6 are 14-bit, 2 MSPS analog-to-digital converters (ADCs), with differential and single-ended inputs, respectively. The devices operate at a 2 MSPS sample rate with a standard 16-clock data frame.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam during storage or handling to prevent electrostatic damage to the MOS gates.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com ELECTRICAL CHARACTERISTICS: ADS7945 (Differential) Minimum/maximum specifications at TA = –40°C to +125°C, AVDD = 2.7 V to 5.25 V, DVDD = 1.65 V to AVDD, input common-mode = VREF/2 ± 0.2, and fSAMPLE = 2 MSPS, unless otherwise noted. Typical specifications at TA = +25°C, AVDD = 5 V, DVDD = 1.8 V, input common-mode = VREF/2 ± 0.2, and fSAMPLE = 2 MSPS. PARAMETER TEST CONDITIONS MIN TYP MAX UNITS –VREF VREF V AIN0P, AIN1P –0.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com ELECTRICAL CHARACTERISTICS: ADS7945 (Differential) (continued) Minimum/maximum specifications at TA = –40°C to +125°C, AVDD = 2.7 V to 5.25 V, DVDD = 1.65 V to AVDD, input common-mode = VREF/2 ± 0.2, and fSAMPLE = 2 MSPS, unless otherwise noted. Typical specifications at TA = +25°C, AVDD = 5 V, DVDD = 1.8 V, input common-mode = VREF/2 ± 0.2, and fSAMPLE = 2 MSPS.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com ELECTRICAL CHARACTERISTICS: ADS7946 (Single-Ended) Minimum/maximum specifications at TA = –40°C to +125°C, AVDD = 2.7 V to 5.25 V, DVDD = 1.65 V to AVDD, and fSAMPLE = 2 MSPS, unless otherwise noted. Typical specifications at TA = +25°C, AVDD = 5 V, DVDD = 1.8 V, and fSAMPLE = 2 MSPS. PARAMETER TEST CONDITIONS MIN TYP MAX UNITS 0 VREF V AIN0, AIN1 –0.2 AVDD + 0.2 V AIN0GND, AIN1GND –0.2 0.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com ELECTRICAL CHARACTERISTICS: ADS7946 (Single-Ended) (continued) Minimum/maximum specifications at TA = –40°C to +125°C, AVDD = 2.7 V to 5.25 V, DVDD = 1.65 V to AVDD, and fSAMPLE = 2 MSPS, unless otherwise noted. Typical specifications at TA = +25°C, AVDD = 5 V, DVDD = 1.8 V, and fSAMPLE = 2 MSPS. PARAMETER TEST CONDITIONS MIN TYP MAX UNITS 2.7 3.3 5.25 V 1.65 3.3 AVDD 3.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com PARAMETER MEASUREMENT INFORMATION TIMING DIAGRAM: ADS7945, ADS7946 Sample N Sample N+1 1/fSAMPLE tACQ tCONV CS tSU1 1 SCLK tWH 2 3 4 tD1 D13 SDO tWL 5 6 7 tH1 D12 D11 8 9 10 tW1 tD4 11 12 13 15 14 16 tD3 tD2 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 Data from Sample N - 1 Table 1.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com PIN CONFIGURATION ADS7945 (DIFFERENTIAL) GND 1 AVDD 2 DVDD SDO SCLK CS 16 15 14 13 RTE PACKAGE QFN-16 (TOP VIEW) 12 PDEN 11 CH SEL ADS7945 8 NC AIN1P 9 7 4 AIN1N REFGND 6 NC AIN0N 10 5 3 AIN0P REF Table 2. PIN FUNCTIONS 8 PIN NO.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com PIN CONFIGURATION ADS7946 (SINGLE-ENDED) GND 1 AVDD 2 DVDD SDO SCLK CS 16 15 14 13 RTE PACKAGE QFN-16 (TOP VIEW) 12 PDEN 11 CH SEL ADS7946 8 NC AIN1 9 7 4 AIN1GND REFGND 6 NC AIN0GND 10 5 3 AIN0 REF Table 3. PIN FUNCTIONS PIN NO.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com TYPICAL CHARACTERISTICS: ADS7945 At TA = +25°C, AVDD = 5 V, DVDD = 1.8 V, VREF = 2.5 V, and fSAMPLE = 2 MSPS, unless otherwise noted. DNL vs ANALOG SUPPLY VOLTAGE DNL vs REFERENCE VOLTAGE 1.5 1 Differential Nonlinearity (LSB) Differential Nonlinearity (LSB) 1.5 Maximum DNL 0.5 0 Minimum DNL −0.5 1 Maximum DNL 0.5 0 Minimum DNL −0.5 AVDD = 5.25 V −1 2.7 3.2 3.7 4.2 4.7 5.2 AVDD, Analog Supply Voltage (V) −1 2.5 5.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com TYPICAL CHARACTERISTICS: ADS7945 (continued) At TA = +25°C, AVDD = 5 V, DVDD = 1.8 V, VREF = 2.5 V, and fSAMPLE = 2 MSPS, unless otherwise noted. OFFSET ERROR vs REFERENCE VOLTAGE 4 3 3 2 2 Offset Error (LSB) Offset Error (LSB) OFFSET ERROR vs ANALOG SUPPLY VOLTAGE 4 1 0 −1 1 0 −1 −2 −2 −3 −3 AVDD = 5.25 V −4 2.7 3.2 3.7 4.2 4.7 5.2 AVDD, Analog Supply Voltage (V) −4 2.5 5.7 3 G012 Figure 7. 5 5.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com TYPICAL CHARACTERISTICS: ADS7945 (continued) At TA = +25°C, AVDD = 5 V, DVDD = 1.8 V, VREF = 2.5 V, and fSAMPLE = 2 MSPS, unless otherwise noted. SNR vs ANALOG SUPPLY VOLTAGE SNR vs REFERENCE VOLTAGE 85 Signal−to−Noise Ratio (dB) Signal−to−Noise Ratio (dB) 85 83 81 79 77 83 81 79 77 AVDD = 5.25 V fIN = 20 kHz fIN = 20 kHz 75 2.7 3.2 3.7 4.2 4.7 5.2 AVDD, Analog Supply Voltage (V) 75 2.5 5.7 3 3.5 4 4.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com TYPICAL CHARACTERISTICS: ADS7945 (continued) At TA = +25°C, AVDD = 5 V, DVDD = 1.8 V, VREF = 2.5 V, and fSAMPLE = 2 MSPS, unless otherwise noted. SINAD vs FREE-AIR TEMPERATURE SINAD vs INPUT FREQUENCY 85 Signal−to−Noise and Distortion (dB) Signal−to−Noise and Distortion (dB) 85 83 81 79 77 fIN = 20 kHz 75 −40 −25 −10 5 20 35 50 65 80 Free-Air Temperature (°C) 95 83 81 79 77 75 110 125 0 20 G023 Figure 19.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com TYPICAL CHARACTERISTICS: ADS7945 (continued) At TA = +25°C, AVDD = 5 V, DVDD = 1.8 V, VREF = 2.5 V, and fSAMPLE = 2 MSPS, unless otherwise noted. SFDR vs ANALOG SUPPLY VOLTAGE SFDR vs REFERENCE VOLTAGE 100 Spurious Free Dynamic Range (dB) Spurious Free Dynamic Range (dB) 100 96 92 88 84 fIN = 20 kHz 80 2.7 3.2 3.7 4.2 4.7 5.2 AVDD, Analog Supply Voltage (V) 96 92 88 84 AVDD = 5.25 V fIN = 20 kHz 80 2.5 5.7 3 3.5 4 4.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com TYPICAL CHARACTERISTICS: ADS7945 (continued) At TA = +25°C, AVDD = 5 V, DVDD = 1.8 V, VREF = 2.5 V, and fSAMPLE = 2 MSPS, unless otherwise noted. ANALOG SUPPLY CURRENT (Dynamic) vs FREE-AIR TEMPERATURE ANALOG SUPPLY CURRENT (Dynamic) vs SAMPLE RATE 4 AVDD Supply Current (mA) AVDD Dynamic Current (mA) 5 4.5 4 3.5 3 3 2.5 AVDD = 5 V 2 1.5 1 AVDD = 3 V 0.5 2.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com TYPICAL CHARACTERISTICS: ADS7945 (continued) At TA = +25°C, AVDD = 5 V, DVDD = 1.8 V, VREF = 2.5 V, and fSAMPLE = 2 MSPS, unless otherwise noted. SPECTRAL RESPONSE 0 −20 Amplitude (dB) −40 −60 −80 −100 −120 −140 −160 0 250 500 750 fIN, Input Frequency (kHz) 1000 G034 Figure 37.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com TYPICAL CHARACTERISTICS: ADS7946 At TA = +25°C, AVDD = 5 V, DVDD = 1.8 V, VREF = 2.5 V, and fSAMPLE = 2 MSPS, unless otherwise noted. DNL vs ANALOG SUPPLY VOLTAGE DNL vs REFERENCE VOLTAGE 2 Differential Nonlinearity (LSB) Differential Nonlinearity (LSB) 2 1.5 Maximum DNL 1 0.5 0 Minimum DNL −0.5 1.5 1 Maximum DNL 0.5 0 Minimum DNL −0.5 AVDD = 5.25 V −1 2.7 3.2 3.7 4.2 4.7 5.2 AVDD, Analog Supply Voltage (V) −1 2.5 5.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com TYPICAL CHARACTERISTICS: ADS7946 (continued) At TA = +25°C, AVDD = 5 V, DVDD = 1.8 V, VREF = 2.5 V, and fSAMPLE = 2 MSPS, unless otherwise noted. OFFSET ERROR vs REFERENCE VOLTAGE 4 3 3 2 2 Offset Error (LSB) Offset Error (LSB) OFFSET ERROR vs ANALOG SUPPLY VOLTAGE 4 1 0 −1 1 0 −1 −2 −2 −3 −3 AVDD = 5.25 V −4 2.7 3.2 3.7 4.2 4.7 5.2 AVDD, Analog Supply Voltage (V) −4 2.5 5.7 3 G049 Figure 44. 5.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com TYPICAL CHARACTERISTICS: ADS7946 (continued) At TA = +25°C, AVDD = 5 V, DVDD = 1.8 V, VREF = 2.5 V, and fSAMPLE = 2 MSPS, unless otherwise noted. SNR vs ANALOG SUPPLY VOLTAGE SNR vs REFERENCE VOLTAGE 85 Signal−To−Noise Ratio (dB) Signal−to−Noise Ratio (dB) 85 83 81 79 77 83 81 79 77 AVDD = 5.25 V fIN = 20 kHz fIN = 20 kHz 75 2.7 3.2 3.7 4.2 4.7 5.2 AVDD, Analog Supply Voltage (V) 75 2.5 5.7 3 3.5 4 4.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com TYPICAL CHARACTERISTICS: ADS7946 (continued) At TA = +25°C, AVDD = 5 V, DVDD = 1.8 V, VREF = 2.5 V, and fSAMPLE = 2 MSPS, unless otherwise noted. SINAD vs FREE-AIR TEMPERATURE SINAD vs INPUT FREQUENCY 85 Signal−to−Noise and Distortion (dB) Signal−to−noise and distortion (dB) 85 83 81 79 77 fIN = 20 kHz 75 −40 −25 −10 5 20 35 50 65 80 Free-Air Temperature (°C) 95 83 81 79 77 75 110 125 0 20 G060 Figure 56.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com TYPICAL CHARACTERISTICS: ADS7946 (continued) At TA = +25°C, AVDD = 5 V, DVDD = 1.8 V, VREF = 2.5 V, and fSAMPLE = 2 MSPS, unless otherwise noted. SFDR vs ANALOG SUPPLY VOLTAGE SFDR vs REFERENCE VOLTAGE 100 Spurious Free Dynamic Range (dB) Spurious Free Dynamic Range (dB) 100 98 96 94 92 90 88 86 84 82 80 2.7 fIN = 20 kHz 3.2 3.7 4.2 4.7 5.2 AVDD, Analog Supply Voltage (V) 98 96 94 92 90 88 86 84 AVDD = 5.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com TYPICAL CHARACTERISTICS: ADS7946 (continued) At TA = +25°C, AVDD = 5 V, DVDD = 1.8 V, VREF = 2.5 V, and fSAMPLE = 2 MSPS, unless otherwise noted. ANALOG SUPPLY CURRENT (Dynamic) vs FREE-AIR TEMPERATURE ANALOG SUPPLY CURRENT (Dynamic) vs SAMPLE RATE 4 4.5 AVDD Supply Current (mA) AVDD Dynamic Current (mA) 5 4 3.5 3 2.5 3 2.5 AVDD = 5 V 2 1.5 1 AVDD = 3 V 0.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com TYPICAL CHARACTERISTICS: ADS7946 (continued) At TA = +25°C, AVDD = 5 V, DVDD = 1.8 V, VREF = 2.5 V, and fSAMPLE = 2 MSPS, unless otherwise noted. SPECTRAL RESPONSE 0 −20 Amplitude (dB) −40 −60 −80 −100 −120 −140 −160 0 250 500 750 fIN, Input Frequency (kHz) 1000 G072 Figure 74.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com OVERVIEW The ADS7945 and ADS7946 are 14-bit, miniature, dual-channel, low-power SAR ADCs. The ADS7945 is a differential input device and the ADS7946 is a single-ended device with ground sensing input. These devices feature very low power consumption at rated speed. The PDEN pin enables an auto power-down mode that further reduces power consumption at lower speeds.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com Figure 76 shows an equivalent circuit of the multiplexer and ADC sampling stage. See the Application Information section for details on the driving circuit. The positive and negative/ground sense inputs are separately sampled on 32 pF sampling capacitors. The multiplexer and sampling switches are represented by an ideal switch in series with a 12 Ω resistance.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com REFERENCE The ADS7945/6 use an external reference voltage during the conversion of a sampled signal. The devices switch the capacitors used in the conversion process to the reference terminal during conversion. The switching frequency is the same as the SCLK frequency. It is necessary to decouple the REF terminal to REFGND with a 1 µF ceramic capacitor in order to get the best noise performance from the device.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com ADC TRANSFER FUNCTION The ADS7945 is a differential input device and the ADS7946 is a single-ended input device. This section describes the transfer characteristics for both devices. The ADS7945 output is in twos compliment format. Figure 79 shows the ideal transfer characteristics for these devices. Here, full-scale range for the ADC input (AINxP – AINxN) is equal to twice the reference input voltage to the ADC 2 × (VREF).
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com The ADS7946 output is in straight binary format. Figure 80 shows ideal transfer characteristics for this device. Here, FSR is the full-scale range for the ADC input (AINx – AINxGND) and is equal to the reference input voltage to the ADC (VREF). 1 LSB is equal to (VREF/2N), where N is the resolution of the ADC (N = 14 for the ADS7946).
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com DEVICE OPERATION The ADS7945/6 operate with either a 16-clock frame or 32-clock frame for ease of interfacing with the host processor. 16-CLOCK FRAME Figure 81 shows the devices operating in 16-clock mode. This mode is the fastest mode for device operation. In this mode, the devices output data from previous conversions while converting the recently sampled signal.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com CS can be held low past the 16th falling edge of SCLK. The devices continue to output recently converted data starting with the 16th SCLK falling edge. If CS is held low until the 30th SCLK falling edge, then the devices detect 32-clock mode. Note that the device data from recent conversions are already output with no latency before the 30th SCLK falling edge.
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ADS7945 ADS7946 www.ti.com SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 conversions complete. However, the devices can still output data as per the timings described previously. The devices consume dynamic power-down current (IPD-DYNAMIC) during data out operations. It is recommended to stop the clock after the 32nd SCLK falling edge to further save power down to the static power-down current level (IPD-STATIC). The devices power up again on the SCLK rising edge.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com In some applications, data collection is accomplished in burst mode. The system powers down after data collection. 16-clock mode is convenient for these applications. Figure 84 and Figure 85 detail power saving in 16-clock burst mode.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com APPLICATION INFORMATION: ADS7945 The ADS7945 employs a sample-and-hold stage at the input; see Figure 76 for a typical equivalent circuit of a sample-and-hold stage. The device connects a 32 pF sampling capacitor during sampling. This configuration results in a glitch at the input terminals of the device at the start of the sample.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com DRIVING AN ADC WITHOUT A DRIVING OP AMP For some low input signal bandwidth applications, such as battery power monitoring or mains monitoring, it is not required to operate an ADC at high sampling rates. In fact, it is desirable to avoid using a driving op amp from a cost perspective. In these cases, the ADC input sees the impedance of the signal source (such as a battery or mains transformer).
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com Table 4 lists the recommended bypass capacitor values and the filter time constant for different source resistances. It is recommended to use a 10 pF bypass capacitor, at minimum. Table 4 assumes RS = 5 Ω; however, depending on the application, RS can be chosen to be 0 Ω. In this case, there is an extra margin of 5 Ω for RSOURCE. Table 4.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com APPLICATION INFORMATION: ADS7946 The ADS7946 employs a sample-and-hold stage at the input; see Figure 76 for a typical equivalent circuit of a sample-and-hold stage. The device connects a 32 pF sampling capacitor during sampling. This configuration results in a glitch at the input terminals of the device at the start of the sample.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com DRIVING AN ADC WITHOUT A DRIVING OP AMP For some low input signal bandwidth applications, such as battery power monitoring or mains monitoring, it is not required to operate an ADC at high sampling rates. In fact, it is desirable to avoid using a driving op amp from a cost perspective. In this case, the ADC input sees the impedance of the signal Equation 4 source (such as a battery or mains transformer).
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com Table 6 lists the recommended bypass capacitor values and the filter time constant for different source resistances. It is recommended to use a 10 pF bypass capacitor (minimum). Table 6. Filter Time Constant versus Source Resistance RSOURCE (Ω) RSOURCE + RS APPROXIMATE CBYPASS (pF) CBYPASS + CSAMPLE (pF) FILTER TIME CONSTANT (ns) 23 28 220 252 7.2 49 54 100 132 7.2 86 91 47 79 7.2 166 171 10 42 7.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com PCB LAYOUT/SCHEMATIC GUIDELINES: ADS7945 The ADS7945 is a mixed-signal device. For maximum performance, proper decoupling, grounding, and proper termination of digital signals are essential. Figure 90 shows the essential components around the ADC. All capacitors shown are ceramic. These decoupling capacitors must be placed close to the respective signal pins. There is a 47 Ω source series termination resistor shown on the SDO signal.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com A common ground plane for both analog and digital often enables better results. Typically, the second PCB layer is the ground plane. The ADC ground pins are returned to the ground plane through multiple vias (PTH). It is a good practice to place analog components on one side and digital components on other side of the ADC (or ADCs). All signals must be routed, assuming there is a split ground plane for analog and digital.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com PCB LAYOUT/SCHEMATIC GUIDELINES: ADS7946 The ADS7946 is a mixed-signal device. For maximum performance, proper decoupling, grounding, and proper termination of digital signals are essential. Figure 92 shows the essential components around the ADC. All capacitors shown are ceramic. These decoupling capacitors must be placed close to the respective signal pins. There is a 47 Ω source series termination resistor shown on the SDO signal.
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ADS7945 ADS7946 SBAS539B – JUNE 2011 – REVISED SEPTEMBER 2011 www.ti.com A common ground plane for both analog and digital often enables better results. Typically, the second PCB layer is the ground plane. The ADC ground pins are returned to the ground plane through multiple vias (PTH). It is a good practice to place analog components on one side and digital components on other side of the ADC (or ADCs). All signals must be routed, assuming there is a split ground plane for analog and digital.
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PACKAGE OPTION ADDENDUM www.ti.
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PACKAGE OPTION ADDENDUM www.ti.com 5-Dec-2013 Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties.
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PACKAGE MATERIALS INFORMATION www.ti.com 14-Jul-2012 TAPE AND REEL INFORMATION *All dimensions are nominal Device Package Package Pins Type Drawing ADS7945SRTER WQFN RTE 16 SPQ Reel Reel A0 Diameter Width (mm) (mm) W1 (mm) B0 (mm) K0 (mm) P1 (mm) W Pin1 (mm) Quadrant 3000 330.0 12.4 3.3 3.3 1.1 8.0 12.0 Q2 ADS7945SRTET WQFN RTE 16 250 180.0 12.4 3.3 3.3 1.1 8.0 12.0 Q2 ADS7946SRTER WQFN RTE 16 3000 330.0 12.4 3.3 3.3 1.1 8.0 12.
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PACKAGE MATERIALS INFORMATION www.ti.com 14-Jul-2012 *All dimensions are nominal Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm) ADS7945SRTER WQFN RTE 16 3000 367.0 367.0 35.0 ADS7945SRTET WQFN RTE 16 250 210.0 185.0 35.0 ADS7946SRTER WQFN RTE 16 3000 367.0 367.0 35.0 ADS7946SRTET WQFN RTE 16 250 210.0 185.0 35.
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IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and complete.