Hi all!
I am using an AD620 (datasheet link) to amplify the output from a pressure sensor: Honeywell 24PCAFA6G (datasheet link). The following are the details of the setup and a picture of my setup. The Vout-Vref from the amplifier stays at a constant value even when the inputs vary from 0 to 4 mV. What am I doing wrong here?
Power supply to the pressure sensor and the instrumentation amplifier AD620 is from a 9V battery.
The gain resistor is 1 kOhm (for a gain of approximately 50)
The input voltage to the amplifier (i.e. the output voltage from the pressure sensor) ranges from 0 - 4 mV.
However, the Vout - Vref reading from pins 5 and 6 of the amplifier stay constant at around 100 mV in the readout from the ADC. What could be the issue?

 

hi FFA,
Are you sure that the IC is an AD620, we have had a couple of threads recently where the AD620 was a remarked 741 OPA.??
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BTW: One way to check is to wire up the IC as a standard OPA and see if it works in that configuration.

@FlyingFordAnglia

How is pin #5 Vref, connected?

 

Can't help you without an accurate schematic of exactly what you have.

 

Hi FFA,
This is what LTSpice shows for your circuit.
E

 

hi FFA,
Are you sure that the IC is an AD620, we have had a couple of threads recently where the AD620 was a remarked 741 OPA.??
E


BTW: One way to check is to wire up the IC as a standard OPA and see if it works in that configuration.

@FlyingFordAnglia

How is pin #5 Vref, connected?

I am sure it is an AD620? We just bought it from DigiKey and the writing on the IC says AD620ANZ. How would I test if it is a remarked OPA?
The pin #5 Vref and pin #6 Vout are both connected to the BNC cable in the picture, which is plugged in to an ADC for readout. pin #5 is connected to the black wire from the BNC and pin #6 to the red one from the BNC. Apart from that pin # 5 is not connected to anything else. Is that what you were asking about?
I am not sure how to wire up the IC as a standard OPA (I am a biologist setting up a sensor for my lab and have minimal experience dealing with electronics).

 

Hi FFA,
This is what LTSpice shows for your circuit.
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View attachment 313500

Thank you for your prompt reply! So it looks like if Va-Vb is 4 mV, Vo-Vr must around 200 mV, which is what I would have expected from the gain of 50. I am not sure why I only get a constant Vo-Vr at around 100mV.

 

Can't help you without an accurate schematic of exactly what you have.

Hope this helps!

 

Hi FAA,
How are the AD620 Vout and Vref outputs, connected to the ADC,?
Usually, the Vref is connected to 0v, or a Vref voltage.
E

 

Try connecting the Ref pin5 to ground and measure output from pin 6 to ground.
What Eric just said.
Why the 5.6K resistor in series with the sensor?

 

Okay, I will try connecting the Vref to ground and see what comes of it. Thank you all for your replies!

 

Try connecting the Ref pin5 to ground and measure output from pin 6 to ground.
What Eric just said.
Why the 5.6K resistor in series with the sensor?

The datasheet for the pressure sensor specified a minimum and maximum for an input and output resistor, so I chose a 5.6 KOhm resistor. But midway through I doubted my interpretation of those terms so I did not connect an output resistor. I guess the resistor in series with the pressure sensor is now vestigial.

 

This is what LTSpice shows for your circuit.

I don't see where Vr is connected(?).

Hope this helps!

It does.
The device does not have rail-rail output and can thus go only to withing about 1.5V of the plus and minus rails, and you show only one supply with no ground reference.
Without a ground reference there is no place for the output current to go.

So you need either need to split the supply using two equal small value resistors in series with decoupling, to generate a pseudo-ground at 1/2 the supply voltage, or use two 9V batteries with the the center connection to ground.

In either case connect REF to ground.

 

I don't see where Vr is connected(?).

Hi C,
This is why I asked him the question regarding the ADC connection, post #8

How are the AD620 Vout and Vref outputs, connected to the ADC,?
Usually, the Vref is connected to 0v, or a Vref voltage.

It is possible that the AD620 Vref is connected to the Vref output of his ADC input or to ground 0V on the ADC.

E

 

I meant where is it connected in your simulation?

 

Hi C,
The TS posted:

The Vout-Vref from the amplifier stays at a constant value even when the inputs vary from 0 to 4 mV. What am I doing wrong here?

Also, his PCB photo shows the lead to the ADC is disconnected and is using a DMM to measure between Vout-Vref
So my basic sim is showing the same result, so I queried and advised to connect the Vref.

How are the AD620 Vout and Vref outputs, connected to the ADC,?
Usually, the Vref is connected to 0v, or a Vref voltage.

E

 

Hi all!
Updates on the issue:
I used two power supplies instead of one, grounded the midpoint of the voltage difference across the two batteries, and connected Vref (pin #5) to the ground. This solved my issue and I am now getting an amplified signal with the gain I set. Here's the corrected circuit for reference. Thank you all for your replies!

 

Hi FFA,
Added the Bridge 0V.
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BTW: Why are you using a 5k6 series resistor in the bridge supply?

 

I am now getting an amplified signal with the gain I set.

Glad in now works.
Isn't it amazing how an IC works properly when you connect it to meet its spec sheet requirements.