• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 47 subscribers
  • Views 738 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LMP91200: VOUT - VCM consistently off by a few mV - Why?

Ronald Roth
Prodigy 50 points
Part Number: LMP91200
Other Parts Discussed in Thread: LM340

I am prototyping a pH measurement circuit on a breadboard.  I have the circuit functionally working, but I have a somewhat consistent error that appears in my measurement, and it is coming from my LMP91200.

In order to remove as many influences as possible, I have disconnected the output of the LMP91200 from any load except my voltmeter.  I have also used a low-impedance source to apply the input voltage to the LMP91200.  I've used both a lab bench power supply  (Rigol DP831) and a 9V battery with a resistor divider.  Both give the results described below.

When I place a voltage across the INP/VCM pin, I expect that the same voltage (albeit buffered) is developed across the VOUT/VOCM pins.  However, I find that there is about a 4mV error, with the VOUT/VOCM pins being below the input voltage.

I varied the voltage from -0.5V to +0.5V, and I measured the voltage at the input and at the output.  The circuit is shown below.  VREF is set at 1.024 V (measured at 1.032 - the precise voltage is not critical to this question)  The graph below shows the results - I am getting a consistent 3-5 mV drop across most of the measurement range.  Everything is measured using recently calibrated voltmeters.  Swapping the meters yields the same results.

Although no bypass caps are shown in the schematic, I did try adding them and it had no effect.  (I only tested a few voltages, not the entire range).

I also tried replace the chip.  Behavior is identical. 

Can anyone advise me on why this voltage drop is appearing?  Although I could add a fudge factor in my calculations, I am very nervous to base a design on this chip if I can't get it to perform as the datasheet claims.  What am I doing wrong?

Thank you,

-Ron

  • 0
    TI__Genius 12780 points

    Hi Ron,

    Unfortunately, the images did not post along with the rest of your message. Could you please try to post the images again?

    Something I would take a look at is using the guard ring inputs to the LMP91200. This technique is used to reduce leakage current when using a high input impedance. Please take a quick look at this design that TI released. Page 18 speaks to how the guard ring is used.

    In the case of using a lab supply to emulate a pH sensor, you may need to supply that same voltage to the guard ring inputs as well.

    Best Regards,

    Dan

  • 0 in reply to dBrock
    Prodigy 50 points

    Dan,

    Thanks for the reply.  Hopefully the pictures come through in this post.

    I'll take a look at your attachment.

    I thought the guard rings are driven by the IC - I don't think it would be wise to drive them externally.  The block diagram on pg1 of the datasheet shows the guard pins being driven by the difference amplifier.

    I tried connecting the guard pins to each other and it had no effect.  On a breadboard, of course, I have no way to modify the layout, but the signal trace is naturally placed between the guard traces.

    Other things I've tried since my last post:

    I replaced the 5V power supply input with a 5-V LM340 voltage regulator powered from 12 VDC.  No effect.

    I used VDD as the VREF input - no effect.

    I grounded the VREF input - this greatly reduced the error (to about 0.6 mV).  However, I need a non-zero VCM, so my VREF cannot be zero.

    I added a 10uF cap between the output pins.  This reduced the error as well - but as soon as I hook it up to the next stage in my circuit, the error comes back.

  • 0 in reply to Ronald Roth
    TI__Genius 12780 points
    Hi Ron,

    Can you try sweeping VREF from 0V o 1.024V and see if the error scales with the increase in VREF? Please try using both +3.3VDD and +5VDD for this sweep.

    Best Regards,

    Dan