SRS Labs SR445A, Manual

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2. Adjust P102 (Channel 1 HF compensation pot) to match the output rise time and
overshoot to the input rise time and overshoot.
3. Repeat for the remaining three channels, adjusting the pulse response with P202,
P302 or P402.
Offset Calibration
The output offset is affected by the HF compensation and so the offset should be nulled
after the HF compensation is adjusted. The offset may also be affected by the amplifier
gain adjustment if there is a large input offset voltage.
1. Leave the inputs unconnected.
2. Connect the output (without a 50 Ω load) to the DVM on the mV dc range.
3. Adjust the offset pot (P101 for channel 1) to null the output voltage.
4. Verify that the output voltage shifts down by less than 2.5mV when a 50 Ω
terminator is placed on the input. (The voltage shift will be 10×[the input bias
current]×[the change of input resistance=25
Ω ] hence this measurement confirms
that the input bias current is less than 10µA.)
5. Repeat the procedure to null the output of channels 2, 3 and 4 by adjusting P201,
P301 and P401.
Gain Calibration
The overall gain of the amplifier is 5× when driving a 50
Ω load and 10× when driving a
high impedance load. The input source to the amplifier is typically a current source (such
as the output from a photomultiplier) and so the magnitude of the input resistance is
included in the gain calibration by calibrating the amplifier’s transconductance.
(Calibration is done with a current source as an input while measuring the voltage at the
output.)
A DVM used in the 4-wire resistance mode is convenient for performing the calibration.
Typically a DVM will measure small resistances by measuring the voltage across the
resistor while passing a 1mA dc current through the resistor. We measure the gain of the
amplifier by measuring the voltage at the output while applying test current to input.
When the gain is properly adjusted, 1mA applied to the 50
Ω input generates 50mV at the
input and 500mV at the (unterminated) output causing the DVM to indicate a
“resistance” of 500
Ω . (To avoid auto ranging “confusion” by the DVM, a 453 Ω resistor
is placed in series with the current source.)
Since the DVM uses a dc current as the test source, it is important that the amplifier
offset be nulled prior to performing the dc gain adjustment.
1. Setup the DVM in the 4-wire resistance measurement mode.
2. Apply the current output to the amplifier input via a 453 Ω in-line resistor.
3. Apply the unterminated amplifier output to the DVM’s 4-wire sense input.