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AMC1100 Gain too high

Mark Beeson
Prodigy 60 points
Other Parts Discussed in Thread: AMC1100, ADS1014

I am using the AMC1100 in an industrial instrumentation device and I am generally very impressed with the product (Survived 7kV surge testing in my circuit)

We have observed something odd though. In the attached circuit the AMC1100 are exhibiting a gain of around 8.3, We have had 5 prototype boards made from the same batch of chips, and they all display the same behaviour. Luckily for me the ADS1014 that sits downstream of the AMC1100 can sample up to the 3.3V supply rail, so I still make the product spec in terms of usable range. There seems to be no other issues. Linearity and offset both appear within spec.

 

Our scope is not very accurate, so the following readings were taken at DC using a stable power supply and a calibrated volt meter.

Short the AMC1100 input together, measure the differential output offset. We read 6.3mV, high, but in spec.

Apply +100mV across the inputs. Expecting 806.3 mV at the output, however we get 836mV,

Apply -100mV accross the input. Expecting -793.7mV at the output, however we get -824mV.

Supply rails are correct and stable.

Any ideas? There is simple LPF between the AMC1100 and the ADS1014. The minimum input impedance of the ADS1014 is about 70k Ohm.

 

  • 0
    TI__Guru**** 180791 points

    Hi Mark,

    Welcome to the E2E forum! I'm happy to hear that you are getting good results so far using the AMC1100 and the ADS1014!

    On the AMC1100 gain issue, can you tell me if you see the same ~8.3 gain variance in both your E_MEAS and V_MEAS circuits? Also, can you let me know exactly where you are shorting the inputs together? Is it directly at pins 2-3 or is it back further in the circuits?

  • 0 in reply to Tom Hendrick
    Prodigy 60 points
    Yes, 8.3 on E_MEAS and V_MEAS on all prototype boards. The shorting occurs further back in the circuit, so I acknowledge there is some additional stray offset currents present at Pins 2 & 3 during this procedure. Indeed it drops to 2.3mV when I use a screwdriver to short pins 2 & 3 on the PCB.

    I
  • 0 in reply to Mark Beeson
    TI__Guru**** 180791 points
    Hi Mark,

    Thank you for confirming! I could see the gain being off a little by the size of your shunt - the input to the AMC1100 is ~28k (see Figure 31), but that does not get you up to to the 8.3 level. Can you tell me what the marking on the AMC1100 chip is (or send a snapshot) and I'll look over the QC/production logs to see if this particular lot of devices is running high on the gain specification.
  • 0 in reply to Tom Hendrick
    Prodigy 60 points

    Requested info attached, along with photographic evidence of the phenomenon.

    The ADS1014 agrees with these readings to within 2LSB.

  • 0 in reply to Tom Hendrick
    Prodigy 60 points

    Hi Tom,

    Any progress?

  • 0 in reply to Mark Beeson
    TI__Guru**** 180791 points

    Hi Mark,

    Sorry for the delay - holiday last week and things have gotten a little backed up...  I've talked with our QC folks and there have not been any reports of abnormalities with the AMC1100, so what I would like to do is see if can get a couple of your devices showing the error checked out.  If it's alright with you, I'll contact you through your my.TI e-mail account and we can go from there.

  • 0 in reply to Tom Hendrick
    Prodigy 60 points

    OK, that's a good idea.

    I'll get one off a board and check it in isolation here first, so we don't waste your time if it turns out to be a quirk of our circuit