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PGA280: Current Source Voltage Limit?

Part Number: PGA280
Other Parts Discussed in Thread: PGA281, MUX36D08

Hello TI,

We're using the current sources on the PGA280 with great success to measure an estimate of loop resistance.  It's working very well for loops of less than ~20K Ohm.

As we're approaching higher resistance loops, however, we're finding that the current sources seem to have trouble once their voltage is more than about +/-1.25V or 2.5V total.

Being that they're connected to VSP and VSN (which, in our circuit are +/-18V), I would have expected them to easily drive, say +/-15V for a 30V potential.

Does this seem correct?  I can find nothing in the datasheet that would lead me to believe this should be the case.

To summarize, no matter how much resistance we connect across INP1 and INP2, when F1 and F2 are turned on, the maximum voltage output by the current sources seems to be around 2.5V.

We have also tried F1/G2 and F2/G1, and the result is roughly the same - ~2.5V at the PGA output in GAIN=1

Appreciate any insight.


  • 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.



  • 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!

    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


  • 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?


    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.



  • Zach,

    Thank you for taking the time to look into this.  Well, that's interesting that you're getting different results than we are.  The circuit that I provided is the entire input network, with the exception that there are actually 8 identical networks multiplexed to the PGA280 through MUX36D08's.  We're multiplexing VERY slowly during the resistance check, so it's not an issue with charging the capacitors or anything like that - we're giving them plenty of time to settle.

    I mentioned in my initial post that we had already tried:

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

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

    We have also tried F1/G2 and F2/G1, and the result is roughly the same - ~2.5V at the PGA output in GAIN=1

    So, I'm not inclined to think it's a common mode issue - Switching in one of the "G" switches should drag the CM pretty effectively to (VSON + (Vdrop/2)).

    I think I'm going to spin a quick board that allows me to switch in each section of the input network one piece at a time and see if I can isolate which section is causing the issue.

    I appreciate the time you took, and I'll start a new post if I find anything interesting.

    Best Regards,