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THS4131: Outputs not balanced down to 0.1mV level

Huy Tran1
Prodigy 20 points
Part Number: THS4131
Other Parts Discussed in Thread: THS4561, THS4551, THP210

HI All,

I'm using a THS4131 in differential inputs. differential outputs mode to drive a 24-bit SAR ADC with Vref = 5V and Vocm = 2.5V. I want to have the two outputs balanced down to 0.1mV. A picture of schematic is attached. Vref is derived from a reference voltage chip with output current capacity 30mA.  I use a Fluke voltmeter up to 4 digit accuracy. I have 2 problems encountered.

1. Vocm is derived from a voltage divider. This Vocm, as soon as, connected to the FDA drops to ~2.42V. If I remove the FDA, the reading is a solid 2.500V. I measured the current drawn from the Vocm pins and its 15mA so it can't be the problem with reference voltage chip not able to provide enough current. Furthermore my simulation on TINA TI also shows this same phenomenon. If I use a set of 500Ohm for the voltage divider, Vocm seems to improve a bit better (about 2.46V) but there not unlimited room for these resistor to keep going down. Why does the FDA cause this problem? Is there any other way to get around this? 

2. This is the more important issue. In reality my Vin would be a 100kHz-500kHz, swinging from 0-10V. Hence the 2 differential outputs should swing between 0-5V out of phase. But to test the outputs imbalance/balance, I ground Vin (0V) and I expected to see the difference between Vout- and Vout+ to be -5.000V on the voltmeter. Instead I measured around -4.997V and this is already a big offset for a 24-bit ADC. The set of 4 resistors are 390Ohm 0.1% 

I even tried a set of 499Ohm 0.01% and it does not seem to improve and even make it worse.

I know this issue can be attributed by many things such as the PCB layout, but I'd like to know if there is anything I overlooked? How can I improve this outputs balance error? 

Any feedback is appreciated! Thank you.

Huy

   

  • 0
    TI__Mastermind 25505 points

    Hello Huy,

    1. Thank you for describing your debugging process! I believe the discrepancy you are seeing is due to the internal 30kOhm voltage divider to the supplies to internally set the VOCM if pin is left floating versus your external set reference voltage. That is the reason why it was a good idea you tested lower resistor values; since, lower resistors values vs 30kOhm resistors will get you closer to the correct value. While increasing internal resistors from 30kOhm to for example 1MOhms would lead to the same improvement.

    2. What is your input source and VREF output impedance? A mismatch any point within the FDA circuit would lead to a differential imbalance. At this link is a FDA calculator (Design tools & simulation tab) that is pretty useful for calculating matched impedances especially when using the FDA as a single-ended input. Also I would suggest measuring differentially post your output resistors/load. 

    Thank you,

    Sima 

  • 0 in reply to Sima Jalaleddine
    Prodigy 20 points

    Hi Sima,

    Thank you so much for your response.

    1. I'm quite surprised the internal VOCM resistance is 30kOhm. Isn't Ri_CM is specified 215MOhm on page 8 of the datasheet? Also I figured it's better to add a buffer after the voltage divider, that would fix the problem easily.

    2. My input source (Vin) comes from an upstream circuitry which I cannot share here. But for the debugging process, I isolate Vin from the rest of circuit and ground it. 

    My VREF (5V) comes from ADR435, but I have room to try LTC6652 which I haven't. I see there's a curve of output impedance vs frequency but I don't know how to interpret this graph. Would you please explain to me how I should interpret this graph ? 

  • 0 in reply to Huy Tran1
    TI__Mastermind 25505 points

    Hello Huy,

      1. The specification is for common-mode input pins of the amplifier (VIN+ and VIN-). However, for output common-mode pin (VOCM), this would be in the output section of the electrical table characteristics. Unfortunately, this specification is not listed directly. I was able to find the information via the functional block diagram on page 21:

        Correct we do recommend using a voltage buffer.

       2. No worries if you are unable to share! Feel free to use the FDA calculator for determining the resistor changes, and let us know if you have any questions there after the debugging confirmation. For the reference, it usually has a complex output impedance due to requiring driving a large cap load. It looks like both devices are buffered, but I am looking into how exactly to calculate output impedance I believe the ADR435 has this in equation 3. But, I have to look into this more. I would consider it low in the <20Ohm range. With the isolation of VIN and not considering VREF for now; what is your differential voltage signal? 

    Thank you,
    Sima

  • 0 in reply to Sima Jalaleddine
    Guru 48395 points

    Also, if you need better DC precision consider the THP210 for higher voltage or the THS4561 midrange supplies, THS4551 5V only. All superior DC error terms, but a bit higher noise. Part of the tool Sima mentions will calculate noise, a lot of that will be resistor noise, so the might not really be too much difference between the THS4131 solution and others.