opamp differential amplifier high frequency/loading effect

[[email protected]]

Hi,
I have a simple difference amplifier based on an opamp. I have few
issues related to its deisgn and analysis.
I have to design it for a differential gain of 10. The gain is decided
by the ratio of the resistors used. R2/R1=R4/R3=10. Normally, what is
the guidelines to select range of resistor values? Is this information
provided in the specific opamp datasheet?
Is there some literature that has analysis on the closed loop gain of
the amplifier taking into the consideration sorce impedance, the
loading effect of the feedback, and the load impedance?
Thanks.
Regards,
Kristo

 

[John Larkin]

Hi,
I have a simple difference amplifier based on an opamp. I have few
issues related to its deisgn and analysis.
I have to design it for a differential gain of 10. The gain is decided
by the ratio of the resistors used. R2/R1=R4/R3=10. Normally, what is
the guidelines to select range of resistor values? Is this information
provided in the specific opamp datasheet?
Is there some literature that has analysis on the closed loop gain of
the amplifier taking into the consideration sorce impedance, the
loading effect of the feedback, and the load impedance?
Thanks.
Regards,
Kristo

Too low will load the input signal and ultimately load the opamp
output, and waste power.

Too high will add Johnson noise, input bias current noise, input
offset from bias current, and can add poles to the closed-loop
frequency response and make the mess oscillate.

In a range of situations, the range of resistors might span many
orders of magnitude.

If you need precision or high common-mode rejection, you are further
constrained by the resistor values you can get.

This is, incidentally, a rather interesting lopsided circuit. The
inverting input doesn't affect the ni input, but the ni input affects
the inverting. So the concept of input impedance is sort of tricky.


John

 

[Mook Johnson]

Hi,
I have a simple difference amplifier based on an opamp. I have few
issues related to its deisgn and analysis.
I have to design it for a differential gain of 10. The gain is decided
by the ratio of the resistors used. R2/R1=R4/R3=10. Normally, what is
the guidelines to select range of resistor values? Is this information
provided in the specific opamp datasheet?
Is there some literature that has analysis on the closed loop gain of
the amplifier taking into the consideration sorce impedance, the
loading effect of the feedback, and the load impedance?
Thanks.
Regards,
Kristo

Depends on how fancy the opamp is and the source impedance and the accuracy
requirements. If the source impadance is high (< a few 100 ohms) it is
generally better to use a three opamp instrumentation amp with the front two
as high impedance buffers to the differential amp.

If you only have one opamp to work with use highish resistor values in the
100K range (say 499K and 49.9K max). The amp impedance looking onto the
inputs will be at least 50K. If thats too high you need the three opamp
design or a true blue instrumtation amp.

If the resistor values go too high you could get oscillation. Even with the
499K you could be on the edge and you should put a 47pF or so cap across
them to reduce the gain at high frequencies (above where your're interested)

 

[[email protected]]

Hi,
I am trying to use just a single opamp.
Is there some reference where i can read about the effect of the
feedback and load on the gain/phase of the closed loop amplifier? Is
it that as long as the opamp can fan out the load, the gain and phase
is not affected?
Regards,
kristo

 

[[email protected]]

Hi,
I am trying to use just a single opamp.
Is there some reference where i can read about the effect of the
feedback and load on the gain/phase of the closed loop amplifier? Is
it that as long as the opamp can fan out the load, the gain and phase
is not affected?
Regards,
kristo

You could just download LTspice, free from http://www.linear.com , and
run some simulations with stepped values for feedback and load
resistors, and look at the gain/phase plots.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

 

[Nemo]

Some simple rules of thumb to get you started:

You say R2/R1=R4/R3=10. It's usual to have R2 = R4 and R1 = R3, too.
Makes the analysis easier.

Make the lower valued resistors (R1 and R3) *at least* 10 times the
resistance of the source they're measuring across, ideally 100 times,
and at least 10 kilohms in any case. This reduces their loading effect
on the source (Kirchoff's Law - you need to make the current used for
measurement insignificant, so the thing under test isn't affected).

If the feedback resistors R2 and R4 are surface mount, and are higher
than, let us say 2.2M, you are likely to begin seeing problems with
leakage currents on the PCB (from dirt - these effects are more usual
with small surface mount components, simply because the terminals are
closer together) unless you have a little experience with this kind of
circuit.