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amplifier instability

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tomas632

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Hi Guys,
I have a circuit I am constructing whereby i am feeding a voltage output from a sensor into an adc. However the output voltage swing of the sensor over the range i am using it is not sufficient to increment the adc counts by much so am using a differential amplifier between the two with the voltage output going to the positive pin and the negative pin connected to a potential divider circuit with a pot on one arm of the divider to allow me to control the divider voltage. However the output of my amplifier is not very stable and I'm extremely puzzled as to why. I do see an increase in amplifier output voltage for an increase in input voltage, but when the input settles at a steady state the output still moves around enough to cause inaccuracies even in my 8-bit adc. the amplifier i am using is in the link below

MC1741CP1 pdf, MC1741CP1 description, MC1741CP1 datasheets, MC1741CP1 view ::: ALLDATASHEET :::

Hope you can help

Tom
 
Hi Guys,
I have a circuit I am constructing whereby i am feeding a voltage output from a sensor into an adc. However the output voltage swing of the sensor over the range i am using it is not sufficient to increment the adc counts by much so am using a differential amplifier between the two with the voltage output going to the positive pin and the negative pin connected to a potential divider circuit with a pot on one arm of the divider to allow me to control the divider voltage. However the output of my amplifier is not very stable and I'm extremely puzzled as to why. I do see an increase in amplifier output voltage for an increase in input voltage, but when the input settles at a steady state the output still moves around enough to cause inaccuracies even in my 8-bit adc. the amplifier i am using is in the link below

MC1741CP1 pdf, MC1741CP1 description, MC1741CP1 datasheets, MC1741CP1 view ::: ALLDATASHEET :::

Hope you can help

Tom

hi Tom,
Do you have a circuit to post, it looks as though you are not using the best OPA.
 
Hi Eric,
Thanks for the quick reply, please see attached circuit
Tom
 

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hi,
With the divider pot in that position you will also be changing the gain of the OPA.
The 5V feeding the divider circuit should be well regulated.

You have not shown any component values, what is the signal range from the current sensor.? and how much gain is the OPA set for.?
 
Hi, so should I put a buffer after my 5V divider? The 5v signal is fairly well regulated.
the signal range from the sensor is approx 2.5v-2.52V so only a 20mV change. The gain is set to around 100 to give me a 2V output swing and the divider is set to about 2.48V at the minute. would you say the OPA is suitable?
 
Hi, so should I put a buffer after my 5V divider? The 5v signal is fairly well regulated.
the signal range from the sensor is approx 2.5v-2.52V so only a 20mV change. The gain is set to around 100 to give me a 2V output swing and the divider is set to about 2.48V at the minute. would you say the OPA is suitable?

hi,
If you post the resistor values we can check the circuit for you.
 
the resistors in the leftmost divider: res=10k pot=10k
input resistors to OPA = 2k
res to ground and feedback resistor are both 200k
hope that helps
 
the resistors in the leftmost divider: res=10k pot=10k
input resistors to OPA = 2k
res to ground and feedback resistor are both 200k
hope that helps

hi Tom,
OK, I will look thru it, bbl
I have attached a pdf and an image that should give you a better idea.
 

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hi,
If this is a correct representation of your circuit, the source impedances are well out of balance so the CMRR will be very poor, also are you using high precision resistors around the OPA.?
 

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what effect will the poor CMRR have? I'm not using precision resistors, but have measured there values to make sure they are pretty similar and yes that is the correct config of my circuit. Would you say the divider branch is affecting the circuit? Shall i put a buffer between it and the OPA?
 
what effect will the poor CMRR have? I'm not using precision resistors, but have measured there values to make sure they are pretty similar and yes that is the correct config of my circuit. Would you say the divider branch is affecting the circuit? Shall i put a buffer between it and the OPA?

hi,
Unless you use high precision resistors common node noise will be a problem.
Did you see my image for the OP AMP subtracter.? this will give you an indication of the cause of the problems you are having

Would you like me to post an alternative circuit.??
 
An alternative circuit would be very helpful. Thanks for your help and I have seen the subtracter image, which makes sense to me. I didn't realise such small deviations in resistance would have such a large effect. So the varying output I am seeing is noise? Could you also define "precision resistor" - 1% tolerance?
 
An alternative circuit would be very helpful. Thanks for your help and I have seen the subtracter image, which makes sense to me. I didn't realise such small deviations in resistance would have such a large effect. So the varying output I am seeing is noise? Could you also define "precision resistor" - 1% tolerance?

Hi
A precision resistor is where the marked resistance is very close to the actual resistor eg: 10K +/-0.01% also they usually have a lower noise value.

Look at this option. it uses the MC1741 amps that you have. See the EDIT.!

The 5V must be very stable and noise free.

EDIT:

Rechecking the datasheet for that OPA suggests that its designed for a dual supply also its output voltage range is limited.

Look at the MCP6002 opa
 

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Last edited:
The 741 opamp was designed 42 years ago to use only a 30V supply. Most do not work with a 10V supply and yours is only 5V.
Use a more modern opamp designed to work properly with a supply as low as 5V.
 
Hi, thanks for your reply. Would this be more suitable? I selected this as they are readily available from my departments stores.

LM709 - Operational Amplifier [Obsolete]

it is also pin compatible with the one i am currently using

hi,
The 709 is MUCH older than the 741 which was made to replace the 709...:eek:

You need a rail to rail OPA if you want to power the OPA with 5V and get close to a 0V to +5V output swing..

Whats your location.?
 
the MCP6002 has a max offset voltage of 4.5mV, surely this wont be suitable?

Considering you have already a DC offset requirement of 2.5Vdc, how will this 4.5mV offset make this device unsuitable.????
 
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