Datasheet

IN1
IN2
MICROPROCESSOR
DSP
SCLK
CS
DIN
DOUT
GND
V
A
ADC102S021
LP2950
5V
1 PF
TANT
0.1 PF 0.1 PF1 PF
| |
|
0V
+V
A
- 1.5LSB
0.5LSB
ANALOG INPUT
1LSB = V
A
/1024
ADC CODE
111...111
111...110
111...000
011...111
000...010
000...001
000...000
ADC102S021
SNAS281G –FEBRUARY 2005–REVISED MARCH 2013 www.ti.com
Figure 49. Ideal Transfer Characteristic
TYPICAL APPLICATION CIRCUIT
A typical application of the ADC102S021 is shown in Figure 50. Power is provided, in this example, by the Texas
Instruments LP2950 low-dropout voltage regulator, available in a variety of fixed and adjustable output voltages.
The power supply pin is bypassed with a capacitor network located close to the ADC102S021. Because the
reference for the ADC102S021 is the supply voltage, any noise on the supply will degrade device noise
performance. To keep noise off the supply, use a dedicated linear regulator for this device, or provide sufficient
decoupling from other circuitry to keep noise off the ADC102S021 supply pin. Because of the ADC102S021's low
power requirements, it is also possible to use a precision reference as a power supply to maximize performance.
The four-wire interface is shown connected to a microprocessor or DSP.
Figure 50. Typical Application Circuit
ANALOG INPUTS
An equivalent circuit for one of the ADC102S021's input channels is shown in Figure 51. Diodes D1 and D2
provide ESD protection for the analog inputs. At no time should any input go beyond (V
A
+ 300 mV) or (GND −
300 mV), as these ESD diodes will begin conducting, which could result in erratic operation. For this reason, the
ESD diodes should not be used to clamp the input signal.
The capacitor C1 in Figure 51 has a typical value of 3 pF, and is mainly the package pin capacitance. Resistor
R1 is the on resistance of the multiplexer and track / hold switch, and is typically 500 ohms. Capacitor C2 is the
ADC102S021 sampling capacitor and is typically 30 pF. The ADC102S021 will deliver best performance when
driven by a low-impedance source to eliminate distortion caused by the charging of the sampling capacitance.
This is especially important when using the ADC102S021 to sample AC signals. Also important when sampling
dynamic signals is a band-pass or low-pass filter to reduce harmonics and noise, improving dynamic
performance.
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