100 PA

1k

2.55V

2.5V

+

-

+5V

1k

1k

1k

1k

2.5V

+IN

-IN

OUT

+5V

V

+

100 PA

1k

2.65V

2.55V

+

-

+5V

1k

1k

1k

1k

2.5V

+IN

-IN

OUT

+5V

V

+

LMP8358

SNOSB09B – APRIL 2010 – REVISED MARCH 2013

www.ti.com

Degraded Input Line

Figure 59 shows an example of a degraded connection between the bridge and the +IN pin of the LMP8358.

This fault is shown by the 1 k

Ω resistor placed between the bridge and the LMP8358. To detect this fault use the

1/50 feature by setting MUX[1:0] to 10b to test the +IN pin, POL to 0b, and G[2:0] to 010b for a gain of 50. This

will give an overall gain of 1. Set CUR[2:0] = 101b to inject a 100µA current and read the output voltage (5082x).

Next set MUX[1:0] to 11b and PIN to 1b to test the

−IN pin as shown in Figure 60 and read the output (58C2x). If

the voltages of these two measurements are different a degraded input fault exists.

Figure 59. Degraded Input Line, Step 1

Figure 60. Degraded Input Line, Step 2

Fault Detection Example

Using the fault detection features of the LMP8358 an end product, such as a scale, can periodically test that no

damage has occurred to the system. A routine can be written that could, for example, run on start up, that will

step through the fault detection features shown above and compare the output voltage to a table like that shown

in Table 16. If the circuit shown in Figure 54 is used the values shown in column 2 of Table 16 would show that

the system is working correctly, the values in the columns under the Possible Faults heading would show that

there is a potential problem and that operator attention is needed.

Table 16. Fault Detection Matrix

No Faults

Possible Faults

LMP8358

Possible Cause

Possible Cause

Possible Cause

Register

00 82x

2.5V

Input is out of

0V

+IN shorted to

CMVR

GND

00 C2x

2.5V

Input is out of

−IN shorted to V+

0V

−IN shorted to

CMVR

GND

50 00x

0.61V

+IN Open

0.07V

Inputs shorted

50 03x

4.97V

−IN

0.7V

Inputs shorted

50 82x

2.55V

2.65V*

Degraded +IN line

58 C2x

2.55V

2.55V*

Degraded +IN line

50 82x

2.55V

2.55V*

Degraded

−IN line

30

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