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Deviation MCP3914 ADC

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Freddy:
I am developing a measuring instrument that contains 5 MCP3914s. This is an 8 channel 24 bit ADC. So I have in total 40 AD converters. Now all 40 ADCs measure approximatelly 1.3 times less on all gain settings. I have used the smaller brother the MCP3911 (2 channel 24 bit ADC) more often without problems.

The analog and digital supply voltages are OK: 3.3 volts and no significant noise (<1 mV). The reference voltage is also in order 1.2 V without significant noise.

With a differential voltage of 30 mV on an input (neatly symmetrical around zero, gain on 1x) I would expect the code (30mv / 600mV) * 2 ^ 23 = 419431, but it measures around 322240. This is confirmed by looking at the SPI stream on a oscilloscope.

Another strange thing is that het maximum code 8388607 is reached at an differential input voltage of 770 mV, but i expect that the maximum would be 600 mV corresponding to te reference voltage of 1.2 V.

After initiaslising the configuration registers read as follows:

0x09 : 0x000000
0x0A : 0x000000
0x0B : 0x000000
0x0C : 0xB90000
0x0D : 0xFB6050
0x0E : 0x000040
0x0F : 0xFFE93A
0x10 : 0x000000
0x11 : 0xFFE825
0x12 : 0x000000
0x13 : 0xFFE5AE
0x14 : 0x000000
0x15 : 0xFFE762
0x16 : 0x000000
0x17 : 0xFFEA61
0x18 : 0x000000
0x19 : 0xFFEA1B
0x1A : 0x000000
0x1B : 0xFFECDA
0x1C : 0x000000
0x1D : 0xFFEFE1
0x1E : 0x000000
0x1F : 0xA50000

As you can see the offset calibration is set and used, and the digital gain calibration is set at x1 for now. (Registers 0x0F and higher.)

Does anyone know what can cause the times 1.3 deviation?

Kleinstein:
Those ADCs need a relatively not impedance source, as the sampling is short. With too much impedance the result can be too low, though a factor of 1.3 is a lot.

uer166:
Can you post front-end/filtering circuit? I am using those too for some metrology and interested in ADC's pitfalls etc..

Freddy:
Here is a part of the analog front-end. Yes, the 10 k impedance is a little high, but with a ADC differential impedance of 232 k at a gain setting of x1 is doesn't explain this large deviation. The input circuit is not much different when I used the MCP3911, and that one worked fine.

iMo:
How do you work with those 30mV in the above schematics then? Do you measure 30mV at the 10k? With all that 5Meg resistors wired in?

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