PGA280: Current Source Voltage Limit?

Hi Ben,

Can you provide a detailed schematic of your circuit, showing your PGA280 configured with the test loop?

From my knowledge of the PGA280, I believe the current sources were designed for a simple wire-break test and are not typically relied on for accurate impedance measurements.

Can you provide your register configurations?  Please provide the configuration for registers 0, 3, 4, 6, 7, 10. I'm especially interested in register 4 as this will tell us if any errors are occurring in the device, or if the input current buffers are active.

Thanks,

Zach

Hello Zach,

I've attached a TINA file showing the input.  PGA280 isn't available for modeling, so I've used PGA281 and added some external 95uA current sources.

I know this isn't an exact replica, since the current sources in the PGA280 are behind some ~600 Ohm switches, but it's close enough for demonstration purposes.

This is what I would expect to see (roughly, again because of internal switch resistance) on the output as the loop resistance is varied from 0 to 50K Ohm

But in practice, it looks more like this (I simulated this by adding a voltage Zener clamp to the input)

This isn't really a terrible problem - We're happy being able to measure up to around 20K.  Really just would like to understand the issue so that we can be relatively sure it won't vary massively between lots of PGA280 parts.  We know that the accuracy of the current sources isn't great, but we intend to calibrate it by connecting a known value resistor to the input and determining the exact current supplied by each part's current sources.  All of the prototypes have current sources that are about 85uA - well within the datasheet specification.

Thank you for your help!
Ben

PGA Registers:

0 - 0x18
1 - 0x00
2 - 0x00
3 - 0x01
4 - 0x00
5 - 0x67
6 - 0x60
7 - 0x0C
8 - 0x6F
9 - 0x27
10 - 0x84
11 - 0x94
12 - 0x08

ARO-PCBB-000195_INPUT_CURRENTSOURCE.TSC

Hi Ben,

I tested this using your register settings and the simple circuit configuration shown below. I was able to get a linear response up to 47kΩ before the output stage began to rail (my next largest resistor is 56kΩ).

Perhaps there is something else in your input network that is stealing the current when the common-mode voltage increases due to the increased differential resistance? I don't see anything that could cause this in your TINA circuit as your input diodes are bootstrapped to the Vcm.

Do you have a more detailed schematic of your actual test circuit?

Could there be another diode in the circuit that is getting biased?

Can you verify the Vcm bootstrapping circuit is operating properly?

PGA280_impedance_test.TSC

It is worth noting that I used 1MΩ resistors to set the common-mode voltage, compared to your 5MΩ. With both current sources switched in, for very high common-mode impedances, any offset current between the two sources will have to flow through the large common-mode impedance and could shift the Vcm out of the linear input range. Another option is to switch in only one of the current sources and switch the other input to ground (VSON).

Can you test this again using the following switch configurations?

Test 1: F1 closed, F2 open, G1 open, G2 closed.

Test 2: F2 open, F1 closed, G1 closed, G1 open.

Regards,

Zach