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Interfacing VCA822 as singled ended input from an OpAmp

Monse Lozano
Prodigy 80 points
Other Parts Discussed in Thread: VCA822, OPA378

Hello,

I'm having trouble configuring the input of the VCA822, I am using it as a single ended input and feeding it from a OPA378 OpAmp. My problem is the signal coming out of the OPA378 was riding on a 2.5V DC bias. I added a 22uF cap to cancel the 2.5V bias but the input of the VCA822 floated to -3.3V. Then I added a 12.7k resistor to ground which helped remove the -3.3V offset, but my output signal of the VCA822 is slightly distorted.

Can anyone recommend the best way to interface the OPA378 with the 2.5V DC bias, to the VCA822, and not distorting my signal? The input signal is a slow ECG signal. The image below is my current set-up.

Thanks!

Monse

  • 0
    TI__Genius 13935 points
    Hi Monse,

    What input voltage does the VCA822 have with the 12.7k Resistor? Did you measure the differential voltage on the two inputs? What is the output voltage of the VCA822 with no input signal from the OPA378?

    If the input was floating to -3.3V would it have been better to add the resistor to V+?

    It's usually a good idea to have the same resistance to ground on both inputs because they both have input bias currents.

    Regards,
    Loren
  • 0 in reply to Loren Siebert 1
    TI__Genius 12631 points
    Hello Monse,
    As Loren suggested I believe you are running into headroom issues on the output because of the large 12.7k resistor. The typical bias current of the VCA822 is 19uA. The input voltage (@ dc) at pin 2 is then 19uA*12.7kOhm = 241mV. This is then amplified by a gain of 10 to the output to become approx. 2.5V. The VCA822 can only swing +/-3.8V under +/-5V supplies so you don't really have much room on the output for the desired ECG signal to swing. This can be simulated in TINA spice if you haven't already tried.

    Again, like Loren suggested if you put a 12.7kOhm resistor to GND on pin 6, the voltage due to the bias current drop across the 12.7k resistor becomes a common-mode input voltage which will be canceled by the VCA822. If you do this, remember to also put a large 1uF cap in parallel with the 12.7k resistor on pin 6 to bypass the resistor noise at higher frequencies.

    -Samir