Datasheet

LMP8358
www.ti.com SNOSB09B –APRIL 2010–REVISED MARCH 2013
Table 16. Fault Detection Matrix (continued)
58 C2x 2.55V 2.65V* Degraded −IN line
* The values shown for a degraded input line will vary depending on the resistance in the line. This table uses
the value in Figure 59 and Figure 60, 1kΩ.
START UP AND POWER ON RESET
During power on, 50µs after V
+
− V
−
> 1V the LMP8358 resets the internal register to 0000x. If the digital
supplies and inputs are undefined after the Power On Reset transients could occur which can cause erroneous
data to be written over the default values in the register. The following should be done to prevent this from
happening:
• Bring all supplies up at the same time. All power supplies, analog and digital, should be brought up together
within 40µs so that the supplies are not undefined after the Power On Reset at 50µs. This is easiest done by
tying the VHSER/VLPAR and VLSER/VHPAR pins to the analog supplies. — Parallel Mode
• Immediately after power on, write to the register the value needed for the application. (This is always
recommended.) — Serial Mode
LAYOUT
The LMP8358 is a precision device that contains both analog and digital sections as shown in the Block
Diagram. The PCB should be carefully designed to minimize the interaction between the analog and digital
sections and to maximize the performance of the part. This should include the following:
0.1µF ceramic capacitors should be placed as close as possible to each supply pin. If a digital supply pin is tied
to an analog pin only one 0.1µF capacitor is needed for both pins. A larger 1µF or 10µF capacitor should be
located near the part for each supply.
Digital and analog traces should be kept away from each other. Analog and digital traces should not run next to
each other, if they do the digital signal can couple onto the analog line. The LMP8358 pinout is set up to simplify
layout by not having analog, power, and digital pins mixed together. Pins 1 — 6 are the analog signals, pins 7 —
10 are the power pins, and pins 11 — 14 are the digital signals.
Be aware of the signal and power return paths. The return paths of the analog, digital, and power sections should
not cross each other and the return path should be underneath the respective signal or power path. The best
PCB layout is if the bottom plane of the PCB is a solid plane.
The REFF and REFS pins are connected to the bottom side of the gain resistors of the LMP8358 as shown in
the Block Diagram. Any impedance on these pins will change the specified gain. If the REFF and REFS pins are
to be connected to ground they should be tied directly to the ground plane and not through thin traces that can
add impedance. If the REFF and REFS pins are to be connected to a voltage, the voltage source must be low
impedance. This can be done by adding an op amp, such as the LMP7701, set up in a buffer configuration with
the LMP7701 output connected to REFF, the negative input of the op amp connected to REFS, and the desired
reference voltage connected to the positive input of the op amp as shown in Figure 61.
DIFFERENTIAL BRIDGE SENSOR
Non-amplified differential bridge sensors, which are used in a variety of applications, typically have a very small
differential output signal. This small signal needs to be accurately amplified before it can be used by an ADC.
The high DC performance of the LMP8358 makes it a good choice for use with a differential bridge sensor. This
performance includes low input offset voltage, low input offset voltage drift, and high CMRR. The on chip EMI
rejection filters available on the LMP8358 help remove the EMI interference introduced to the signal as shown in
Figure 61 and improves the overall system performance.
The circuit in Figure 61 shows a signal path solution for a typical bridge sensor using the LMP8358. The typical
output voltage of a resistive load cell is 2mV/V. If the bridge sensor is using a 5V supply the maximum output
voltage will be 2mV/V × 5V = 10mV. The bridge voltage in this example is the same as the LMP8358 and
ADC161S626 supply voltage of +5V. This 10mV signal must be accurately amplified by the LMP8358 to best
match the dynamic range of the ADC. This is done by setting the gain of the LMP8358 to 200 which will give an
output from the LMP8358 of 2V. To use the complete range of the ADC161S626 the V
REF
of the ADC should be
set to half of the input or 1V. This is done by the resistor divider on the V
REF
pin of the ADC161S626. The
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