Opamp question

[Anders Nesheim Vinje]

I was wondering why in an inverting opamp configuration, sometimes there is
resistor from the + input to ground.
The resistor often matches the resistor going into the - input in where the
signal goes trough. Does this improve the overal perfomance of the opamp?
Has this something to do about the matching input impedance and therefore
amplify both the postive and negative part of the singnal equalley and
theirby rejecting more noise??

Anders N. Vinje

 

[John Popelish]

Anders Nesheim Vinje wrote:

I was wondering why in an inverting opamp configuration, sometimes there is
resistor from the + input to ground.
The resistor often matches the resistor going into the - input in where the
signal goes trough. Does this improve the overal perfomance of the opamp?
Has this something to do about the matching input impedance and therefore
amplify both the postive and negative part of the singnal equalley and
theirby rejecting more noise??

The resistor from + input to ground is used only when the opamp bias
current causes an objectionable offset at the opamp inputs when it
passes through the input resistances. If the total parallel
resistance of the network on the - input is matched by a resistor on
the + input, the approximately equal input bias currents will produce
similar voltage drops and there will be a cancellation. The resistor
on the + input actually increases the noise a bit, because the bias
current has some noise in it, and the resistor will drop a variable
voltage as this noisy current pases through it. The resistor also
contributes a bit of its own noise to the input.

 

[Ban]

Anders Nesheim Vinje wrote:

I was wondering why in an inverting opamp configuration, sometimes
there is resistor from the + input to ground.
The resistor often matches the resistor going into the - input in
where the signal goes trough. Does this improve the overal perfomance
of the opamp? Has this something to do about the matching input
impedance and therefore amplify both the postive and negative part of
the singnal equalley and theirby rejecting more noise??

Anders N. Vinje

Anders,
in older opamps you will find that the bias current is much bigger than the
offset current. Then this resistor might be of some impact in high ohmic
circuits.
Lets look at the venerable LM324: Ibias is typically 45nA, whereas Ioffset
is only 5nA. With a 100k source resistance the resistor will reduce the bias
induced offset error from 4.5mV to 0.5mV. Additionally you have to add the
2mV offset voltage. This is input related, a gain in the circuit also
amplifies these offsets.
But many modern precision opamps have some cancellation circuit built in, so
the bias current is no more larger than the offset current. In this case
there will be no improvement.
The OPA177 has 0.5nA bias and 0.3nA offset current. This will cause 50uV
offset in the 100k resistor which could be reduced only to 30uV with the
resistor. This opamp has only 20uV additional offset voltage in the cheap
version.
Of course inserting the resistor has also disadvantages: the source resistor
noise will go up 3dB unless you bypass it with a cap, and also the input is
more prone to induction, because its impedance is higher now.
--
ciao Ban
Bordighera, Italy

 

[John Popelish]

stratus46@yahoo.com wrote:

Question , John,
Would a capacitor from + input to ground reduce the bias and resistor
noise?
Thanks
Glenn Gundlach

For frequencies higher than 2*pi*R*C it would for the resistor and
current noise. It doesn't help the input voltage noise, since that is
effectively a source in series with the capacitor.

 

[tlbs]

The resistor from the + input to ground will act to equalize the errors
due to bias current (Ib). Bias current flows through both the feedback
resistor and the resistor from the - input to ground (through the
impedance of the input source), and generates a small voltage. If
there is no resistor present at the + input, that voltage due to the
bias current produces an error at the output.

By placing a resistor from the + input to ground, the bias current
flowing from the + input generates a voltage across the resistor equal
to that in the negative side and the error voltage cancels out at the
output.

The ideal value for the resistor is: the parallel combination of the
feedback resistor, and the sum of the input resistor with the source
input impedance. Any value other than zero-ohms will reduce the bias
current error, though (as is the case in your example).

 

John Popelish wrote:

Anders Nesheim Vinje wrote:
I was wondering why in an inverting opamp configuration, sometimes
there is
resistor from the + input to ground.
The resistor often matches the resistor going into the - input in
where the
signal goes trough. Does this improve the overal perfomance of the
opamp?
Has this something to do about the matching input impedance and
therefore
amplify both the postive and negative part of the singnal equalley
and
theirby rejecting more noise??

The resistor from + input to ground is used only when the opamp bias
current causes an objectionable offset at the opamp inputs when it
passes through the input resistances. If the total parallel
resistance of the network on the - input is matched by a resistor on
the + input, the approximately equal input bias currents will produce

similar voltage drops and there will be a cancellation. The resistor

on the + input actually increases the noise a bit, because the bias
current has some noise in it, and the resistor will drop a variable
voltage as this noisy current pases through it. The resistor also
contributes a bit of its own noise to the input.

Question , John,
Would a capacitor from + input to ground reduce the bias and resistor
noise?
Thanks
Glenn Gundlach