On Wednesday, April 6, 2016 at 4:47:46 PM UTC-4, Fibo wrote:
> Greeting,
>
> I'm having trouble interfacing an OpAmp (OPA378) to a Voltage Controlled 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 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 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 link shows the current condition of my circuit. Any thoughts are much appreciated.
>
> The VCA has differential input, I'm using it as single ended.
>
> link: https://drive.google.com/open?id=0B57i2560inOARGFMQVRkb0h6WTQ
>
> Much thanks!
Slightly distorted because of the low pass? Then shift it,
more C and/or more R.
George H.
Reply by ●April 8, 20162016-04-08
On Wednesday, April 6, 2016 at 4:47:46 PM UTC-4, Fibo wrote:
> Greeting,
>
> I'm having trouble interfacing an OpAmp (OPA378) to a Voltage Controlled 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 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 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 link shows the current condition of my circuit. Any thoughts are much appreciated.
>
> The VCA has differential input, I'm using it as single ended.
>
> link: https://drive.google.com/open?id=0B57i2560inOARGFMQVRkb0h6WTQ
>
> Much thanks!
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, relying on the OA PSRR to do most of the filtering for you.
Reply by Jon Elson●April 6, 20162016-04-06
Fibo wrote:
> Greeting,
>
> I'm having trouble interfacing an OpAmp (OPA378) to a Voltage Controlled
> 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
> 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 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 link shows the current condition of my circuit. Any
> thoughts are much appreciated.
>
> The VCA has differential input, I'm using it as single ended.
>
> link: https://drive.google.com/open?id=0B57i2560inOARGFMQVRkb0h6WTQ
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 by Phil Hobbs●April 6, 20162016-04-06
On 04/06/2016 05:07 PM, Phil Hobbs wrote:
> On 04/06/2016 04:47 PM, Fibo wrote:
>> Greeting,
>>
>> I'm having trouble interfacing an OpAmp (OPA378) to a Voltage
>> Controlled 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 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 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 link shows the current condition of
>> my circuit. Any thoughts are much appreciated.
>>
>> The VCA has differential input, I'm using it as single ended.
>>
>> link: https://drive.google.com/open?id=0B57i2560inOARGFMQVRkb0h6WTQ
>>
>> Much thanks!
>>
>
> 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
>
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 by Phil Hobbs●April 6, 20162016-04-06
On 04/06/2016 04:47 PM, Fibo wrote:
> Greeting,
>
> I'm having trouble interfacing an OpAmp (OPA378) to a Voltage
> Controlled 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 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 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 link shows the current condition of
> my circuit. Any thoughts are much appreciated.
>
> The VCA has differential input, I'm using it as single ended.
>
> link: https://drive.google.com/open?id=0B57i2560inOARGFMQVRkb0h6WTQ
>
> Much thanks!
>
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 by Fibo●April 6, 20162016-04-06
Greeting,
I'm having trouble interfacing an OpAmp (OPA378) to a Voltage Controlled 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 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 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 link shows the current condition of my circuit. Any thoughts are much appreciated.
The VCA has differential input, I'm using it as single ended.
link: https://drive.google.com/open?id=0B57i2560inOARGFMQVRkb0h6WTQ
Much thanks!