Three Port optical Isolation amplifier

[Godknows]

Hi! I am building a three port optical isolation amplifier. I am using the AD620ANZ Instrumentation amplifier to almost completely remove the common voltage offset. I do not know why I am having this +6.7Vdc at the output of the AD620 and I do not want it. Please help
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[Reloadron]

I can't make out the first part of your drawing? What chip is that and resistor values?

Ron

 

[Godknows]

It is a CA3140 opamp. The resistor values are 1k, respectively. I am using this to create a differential voltage for the AD620ANZ inamp at the moment. The input to the noninverting amplifier is a voltage divider circuit that reduces the input voltage to mvs. Thanks

 

[Reloadron]

You are going to have to take a look at the AD620 inputs, the only way you will have that DC level out is if it is going in and being amplified with your AC signal. You can't use the Vref (pin 5) to remove that much DC.

Ron

 

[Godknows]

Thanks for the reply. But how would the DC be amplified? From where could it have come? Can I use a highpass filter at the output of the amplfier to remove the dc offset?

 

[crutschow]

Measure the voltages at the AD620 input.

 

[Godknows]

You mean in Vdc as well? let's assume that the Vdc in the input got amplified; How can I remove the amplified DC component? Or How can I remove the DC component at the input of the AD620ANZ? Thanks

 

[Tony Stewart]

SHow a block diagram of what you are trying to do.

What I see makes no sense with an input attenuated by large R ratio then inverted thru 1st chip and non inverted and both fed to IA with a gain near 1000. So I expect you have excessive Voffset in 1st stage an no provision for offset null.

 

[Godknows]

Ah. Let me explain the above block diagram. I am not using the ECG signal yet. I want to make it work with an AC signal before I move to ECG signal - the main aim of the work.

The ECG signal is in the millivolt range and I cannot get that range with the Function generator that I am currently using. That is why I am using the large value ratio of R (i.e. Voltage divider) to generate a pseudo millivolt range for the AD620ANZ amplifier.

The CA3140 configuration is a non inverting configuration with a gain of 2 in order to generate another pseudo differential voltages. These differential voltages are fed to the input of the AD620ANZ which gives out the amplified difference of its input.

I do not know why I got that dc component value at its output as I do not see the reason why it is at the output of the AD620ANZ amplifier.

Or is it because the offset null of the CA3140 (pin 5) isn't connected to ground that is why there is this huge dc offset? Thanks

 

[Tony Stewart]

refer your Vo offset to input it is only mV of offset.

drop the CA3140 and measure differential from the tapped signal on the resistor with respect to ground.
Add 100K resistors on each Signal and ground (V+pin3 ) . (EDIT) to simulate electrode impedance range if you wish

But use a large ferrite torroid CM choke as you will dealing with high impedance and use shielded pairs so hum will be a problem.
When both inputs are grounded the output should be silent with no DC offset at a gain of 1000 (2000?)and decoupling caps must be close to chip.

 

[Tony Stewart]

CA3140 input offset is 5mV

 

[Godknows]

Thanks. Isn't the AD620ANZ only differential power supply? Why should I connect the (V-) to ground. If I do so, would I be able to only use 4.5volts Vcc to power the instrumentation amplifier?

 

[Tony Stewart]

my mistake Vin(+) .. remove from CA3144 and use gnd or if you prefer use a common mode voltage,

So 1Vac gen>cap > 1M>10k>1M>1Vdc to simulate 1mVac on 1Vdc and amplify the 1mV across 10k to get desired output with 0V dc with 1Vac out.

In reality there will always be DC on electrodes from galvanic skin response acting as a battery so you may have to use HPF at 1~10Hz.
e,g, 10M load on +_ Vin and 1nF in series with each. for 8Hz

 

[Godknows]

I am confused about this latest reply. How do you mean?

 

[Godknows]

Do you mean pin3 of the AD620ANZ? If yes, is this what you meant?

 

[Tony Stewart]

I was just suggesting a differential attenuator

But you need to consider much more than just an IA for stray hum rejection.,

1) reduce the common mode impedance.
2) use common mode output to drive a leg electrode and null the offset.
3) use 50/60Hz notch filter with null offset

Some use all 3 methods use overvoltage protection.
Most common is 2)

Method 1 with DC offset integrator and single supply below.

Note : DC offset is done with C1 integrator, but AC coupling 0.01Hz still done in last stage.

 

[Godknows]

Thanks. I will go by the first 2 methods since the notch filter has a somewhat negative impact on the signal.
Can you please explain the diagram you presented more?
Is the first section called bootstrapping?
Why the BAV99 diodes?
Are they for reducing the common mode voltage?
What about the MCP507 circuit at the RG and REF pin of the INA321 instrumentation amplifier?
What is it for?
I understand the idea of the bandpass filter at the output of the INA321 as it covers a particular range of frequencies. But what are the other 2 circuit at the front of the bandpass filter for?
The CMREF in the circuit, does it mean common mode reference? if yes, is it vcc/2?

Did you get the good circuit from a textbook or somewhere else? Perhaps, I could go there and also enrich myself with the good knowledge that you have acquired from it.

 

[Tony Stewart]

This is the opposite of bootstrapping which raised input impedance. In this case it lowers CM impedance while maintain high differential impedance. I found on web but not tried it.

RG is driven by integrator to null DC at REF

Essentially active HPF.

Since that cct has very long time constant. startup time may be long , so if necessary a quick startup pulse to null offset ( active clamp) can be considered, like how TV signals are clamped for every row. Depends on implementation for details.

 

[Tony Stewart]

All inputs ought to be designed for 15kV ESD protection, no damage. hence diodes+ R

 

[alec_t]

I do not know why I got that dc component value at its output

The gain of the AD620 is set at ~2000 by the 47 Ohm resistor. If you have ~6VDC at its output that is because the CA3140 has an input offset voltage of ~6/2000 = 3mV.