- Compensation Design Information -
Input Offset Voltage
The most common compensation involves adding a resistor [R3] to the standard inverting amplifier to cancel out bias currents [output offset].
The compensation resistor [R3] causes a current, on the positive terminal, equal and opposite to current flowing into the negative terminal.
So any DC output offset caused by the inverting input is cancelled by the non-inverting input.
The value of R3 should be equal to the parallel combination of R1 [Ri] and R2 [Rf].
Input Parasitic Capacitance
Compensation: The process of overcoming the problems associated with frequencies in an
Compensate for input parasitic Capacitance
A small capacitor [Cf] across the feedback resistor will compensate for parasitic
capacitance at the (inverting) input of the Op-Amp. Normally 3pF to 10pF
will compensate. This is true for Inverting, Non-Inverting, or Voltage
Follower configurations. The capacitor also functions to reduce gain at higher frequencies.
A feed back pole is created when the feedback
around any amplifier is resistive. The parallel resistance and
capacitance from the input of the device (usually the inverting input) to
AC ground set the frequency of the pole. In many cases the frequency of
the pole is much greater than the 3dB frequency of the closed loop gain ~
there is negligible effect on the stability margin.
How ever, if the feed
back pole is less than approximately six times the expected 3dB
frequency, a 'lead' capacitor should be placed from the output to the
input of the Op-Amp.
The value of the added capacitor should be such that the RC time constant of this capacitor and
resistor it parallels is greater than or equal to the original feed back
pole time constant.
Power Supply Inductance
Compensate for input power Supply inductance
Stray capacitance between the power supply terminal and the Op-Amp input
terminal will allow (in phase) current, developed across the power
supply's "inductor" to flow into the input of the Op-Amp. Use low
inductance capacitors to by-pass the power supply leads.
Compensate for driving a large capacitive load
Add a resistor in series with the output terminal of the Op-Amp, between
the output and the feed-back resistor. Make the resistor around the same
value as the output impedance of the Op-Amp.
In addition, a by-pass capacitor may be added directly between the input and output
Compensate for PWB Layout
Resistors from the output to the input should be placed with the body
close to the input to minimize "pick-up" and maximize the frequency of
the feed back pole by minimizing the capacitance from the input to
ground. To control leakage currents in ultra-low current applications run
a guard trace around the (high impedance) input pins and connecting it to
a low impedance source (at the same level).
The guard trace should be added to both sides of the PWB. Unused op-amps in dual and quad packs
should be terminated by grounding the + input and connecting the - input to the output.
Related Engineering Data Operational Amplifier Derating [Temperature Compensation].
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