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Original question:

INA1650: Damage without input decoupling capacitor

INA1650: Question on DC bias

Craig Phillips
Prodigy 20 points
Part Number: INA1650
Other Parts Discussed in Thread: OPA1677, OPA1688

In the datasheet of OPA1677, there's an example circuit (copied below) that illustrates the use of said op-amp to force the output voltage of a INA1650 stage to 0v. However, the input to the INA1650 is AC-coupled, so there should not be any DC going through; shouldn't the voltage of A equal IN+A minus IN- A  ? So maybe I'm missing a piece of the puzzle and I'd like to understand it before I finalize my circuit based on the INA1650.

Now I guess there could be a potential difference between the chassis ground and the circuit ground. If the COM inputs are tied to the chassis ground, as they are in this example via 1M ohm resistor, could that incur a DC offset, and that is what needs to be removed? I couldn't see much in the datasheet regards the COM inputs and I couldn't find the model in TINA for INA1650 to test that. If that is the case, I could just tie the COM pins to local ground instead, but perhaps that would adversely affect CMMR?

  • 0
    TI__Mastermind 21980 points

    Hey Craig, 

    I will run this by another engineer as well but, I don't see a reason either as you mentioned. I believe this idea came from the floating inputs discussion in the INA1650 datasheet. In this case though the inputs have the DC potential set to mid supply or ground because the com pin is set to ground. Therefore they are not floating. I tested this in Tina using the reference design from the INA650 product page. Link below. 

    https://www.ti.com/product/INA1650#design-tools-simulation

    Free Tina Download:

    https://www.ti.com/tool/TINA-TI

    You can see below that the output DC potential is near mid supply. 

    Best Regards, 

    Chris Featherstone

  • 0 in reply to Chris Featherstone
    Prodigy 20 points

    Thanks for your prompt response, Chris. I'll download the INA1650 spice model. The SE to Diff conversion block looks useful too. How do I get that in TINA?

    So if I'm reading your simulation correctly, the B channel is DC coupled and there is still negligible offset at the output. The -500m is a DC mV offset on both signal lines that gets cancelled.

    I think you're right in that the OPA1688 "DC-servo" circuit makes much more sense on floating single-ended inputs as depicted in the INA1650 datasheet. Maybe it's just as simple as that circuit should've been used in the OP1677 too. But I was wondering, given the two different ground symbols used, that if the chassis ground had a different potential to the the local ground, maybe that would appear on the COM input and shift the output?

  • +1 in reply to Craig Phillips
    TI__Mastermind 21980 points

    Hey Craig, 

    I added my Tina sim below so you can use the SE to DIFF conversion. The 500m is just the voltage amplification for the input signal and sends 50% to each leg. The input on B is DC coupled to mid supply or ground. I ran a DC common mode sweep on the com pin. The CMRR of the INA takes care of rejecting common mode shifts. This represents the potential delta you are referring to between the two grounds. I also ran a transient analysis on the sim. 

    INA1650.TSC

    Best Regards,

    Chris Featherstone