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Interfacing an OpAmp to a differential VCA

Greeting,

I'm having trouble interfacing an OpAmp (OPA378) to a Voltage Controlled Am plifier IC (VCA822). My signal coming out of the OPA378 is riding on a 2.5V DC bias. I have a 22uF cap to cancel the 2.5V bias but with the cap there the input of the VCA822 floated to -3.3V. I then added a 12.7k resistor to ground after the cap which helped remove the -3.3V offset, but my output si gnal 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 sl ow ECG signal. The link shows the current condition of my circuit. Any thou ghts are much appreciated.

The VCA has differential input, I'm using it as single ended.

link:

formatting link

Much thanks!

Reply to
Fibo
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Well, one approach would be to use the chopamp noninverting, AC-couple its + input (1 uF in series and 470k to Vref), and send the reference voltage to the other input of the PGA. Use a nice large-size, high voltage cap (1206, 50- or 100 V) to minimize the nonlinearity. Film caps are better than ceramics for coupling jobs, but more expensive and more of a pain--you'll want to use through-hole ones.

Some chopamps are less happy in the noninverting configuration.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Oh, and of course you'll need to run that reference at much higher current. If you don't like the increased drain, you can do the same AC-coupling thing with a second unity-gain chopamp between IN and R3, and leave U1 the way it is.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Your 2.5 V reference could be a noise source bigger than your signals, unless you are using something like an LM4140 reference. either that, you you might need to put a big RC filter after it. The + input of U1 is at

+2.5 V, what DC level is the IN terminal at? Seems like you may need a DC blocking capacitor at that input.

Jon

Reply to
Jon Elson

Amplifier IC (VCA822). My signal coming out of the OPA378 is riding on a 2.

5V DC bias. I have a 22uF cap to cancel the 2.5V bias but with the cap ther e the input of the VCA822 floated to -3.3V. I then added a 12.7k resistor t o ground after the cap which helped remove the -3.3V offset, but my output signal of the VCA822 is slightly distorted.

C bias, to the VCA822, and not distorting my signal? The input signal is a slow ECG signal. The link shows the current condition of my circuit. Any th oughts are much appreciated.

Why are you even using that VCA if you're adjusting the gain with a pot?!? Ignoring that lunacy, the best solution would be to power the OPA378 off of +/-2.5 supplies and eliminate all the DC blocking caps and references, rel ying on the OA PSRR to do most of the filtering for you.

Reply to
bloggs.fredbloggs.fred

Amplifier IC (VCA822). My signal coming out of the OPA378 is riding on a 2.

5V DC bias. I have a 22uF cap to cancel the 2.5V bias but with the cap ther e the input of the VCA822 floated to -3.3V. I then added a 12.7k resistor t o ground after the cap which helped remove the -3.3V offset, but my output signal of the VCA822 is slightly distorted.

C bias, to the VCA822, and not distorting my signal? The input signal is a slow ECG signal. The link shows the current condition of my circuit. Any th oughts are much appreciated.

Slightly distorted because of the low pass? Then shift it, more C and/or more R.

George H.

Reply to
George Herold

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