• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 48 subscribers
  • Views 209 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.

OPA3S328EVM: Using -INA input with ADS1285EVM DAC1282 input

Joseph Joyce
Prodigy 30 points
Part Number: OPA3S328EVM
Other Parts Discussed in Thread: DAC1282, BP-DAC11001EVM, OPA3S328, INA592, INA826

Tool/software:

Hi,

I'm trying to use the ADS1285EVM onboard DAC1282 to output a very small signal as an input to -INA on Site 1 of the OPA3S328EVM.

I then take OUTPUT B of the OPA3S328EVM and feed it back into AIN1 of the ADS1285EVM.

I'm trying to do this for very small voltages, so my DAC1282 gain is 1/64.

My present problem is that when I do this, the measured output of the DAC1282 is incorrect. It is orders of magnitude higher.

In order to troubleshoot this, I have connected the DAC1282 internally on the ADS1285EVM to AIN2 and also wired it directly to AIN1.

Both make correct readings from the evm's ADS and a voltmeter.

I have separately connected the BP-DAC11001EVM to the OPA3S328EVM.

It does not exhibit this problem. I am able to output a voltage to the OPA -INA input and it is correct at the input and output.

However, I cannot continue to use the DAC11001 because it does not have a digital gain that I can use to get the signal granularity that I need.

Some additional info...

I'm using the OPA in bipolar mode.

My V+ (2.5V) and V- (-2.5V) and GND are provided from the ADS1285EVM.


Can you help me troubleshoot this?

My end goal is to read current input with uA granularity.

Best,

Joe

  • 0
    TI__Mastermind 46550 points

    Hello Joe,

    Can you provide a schematic or drawing showing how you have the two EVM boards connected together, including power?  Also, please provide details on the positions of all shunts on the ADS1285EVM (a picture of the board will also work).

    The ADS1285EVM has a -2.5V regulator for AVSS, but in order to use this supply, a negative voltage from a bench supply is required on J3.  (The negative supply is not generated from the USB 5V bus power.)

    Also, since the output of the DAC1282 is differential, the common mode voltage will be set close to mid-supply; if using the default single 5V AVDD supply, the common mode voltage on each of the DAC outputs will be 2.4V, which would be amplified by the OPA3S328 if using a single-ended connection.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 30 points

    Hi Keith,

    Thanks for your response and queries on this.

    I'm attaching pictures of my setup.

    My negative supply is provided via J3 on the ads1285evm. I'm providing -3.0V.

    My positive supply is provided via the red cable and the 5.5V test point. I'm providing 6V to it.

    The OPA3S328EVM V- (the green cable) is provided from the -2.5V reference test point.

    The OPA3S328EVM V+ (the blue cable) is provided from the AVSS+5V reference test point which is +2.5V.

    All of the dangling resistors that you see on the i/o of the 328 are 0R.

    Can you please take a look and advise any improvements?
    Best,

    Joe

  • 0 in reply to Joseph Joyce
    TI__Mastermind 46550 points

    Hello Joe,

    Based on your pictures and description, I think you have the power supplies all connected correctly.  However, the output of the DAC1282 is not directly compatible with the OPA3S328EVM.

    The output of the DAC1282 is differential, and always includes a common mode voltage of -0.1V when operating from +/-2.5V bipolar supplies.

    For example, if you output +10mV, you will measure the following voltages on the DAC+ and DAC- outputs with respect to ground.

    DAC+:  -95mV

    DAC-: -105mV

    The differential output of the DAC will then be:  DAC+ (-) DAC- = +10mV.  The common mode voltage is the average, or [DAC+ (+) DAC-]/2=-100mV.

    If you output -10mV, then you will see the following:

    DAC+:  -105mV

    DAC-: -95mV

    The differential output of the DAC will then be:  DAC+ (-) DAC- = -10mV.  The common mode voltage is the average, or [DAC+ (+) DAC-]/2=-100mV.

    When you connect a single output from the DAC1282, the OPA3S328 will see 1/2 of the DAC differential value plus the common mode.  In the previous example, instead of amplifying 10mV, it is amplifying about 100mV.

    If the DAC1282 had a common mode voltage of 0V, then you could get this to work with the connections, but the DAC1282 is intended to be used with differential inputs that remove this common mode.  The -0.1V is there to optimize the output range of the DAC1282, and cannot be removed by changing the DAC settings.

    Unfortunately, I do not see a way to get this to work with the existing OPA3S328EVM.  I suggest using either a difference amplifier or an instrumentation amplifier to convert the differential output of the DAC1282 to a single-ended output, which will remove the -0.1V offset (The DAC11001 is also a single-ended output, which is why it works correctly with the OPA3S328).  INA592 would be a good instrumentation amplifier and the INA826 would be a good instrumentation amplifier.  Both devices have an EVM available.  You can then use the output of the INA board for your system, or connect this output to the OPA3S328EVM.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Prodigy 30 points

    Thanks for the detailed response Keith. Will take a look at the INA592 and INA826.