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FRONT-END AMPLIFIER
On the high sensitivity scales, since noise is
dominated by DC current offset, an op amp with
a very low input current is used on all gain
modes. On intermediate sensitivity scales, a low
noise op amp is switched in for the LOW
NOISE and HIGH BW modes to improve the
AC response of the front-end. On the less
sensitive scales, where DC drift is dominated by
drift in the offset voltage, the low noise op amp
is used in all gain modes. On all scales, the High
BW mode is obtained by shifting some of the
gain from the front-end to the output stage.
Op Amp Allocation Table
Scale
Low Noise High BW
1 mA/V
1
100 µA/V
1
10 µA/V
1
1 µA/V
1
100 nA/V
1
10 nA/V
2
1 nA/V
2
100 pA/V
2
10 pA/V
2
1 pA/V
2
1 = AD743 (Low Noise)
2 = AD546 (Low Input Current)
DYNAMIC RESERVE
The term "Dynamic Reserve" comes up
frequently in discussions about amplifiers. It's
time to discuss this term in a little more detail.
Assume the amplifier input consists of a full
scale signal at fsig plus noise at some other
frequency. The traditional definition of dynamic
reserve is the ratio of the largest tolerable noise
signal to the full scale signal, expressed in dB.
For example, if full scale is 5 µA, then a
dynamic reserve of 60 dB means noise as large
APPENDIX B
Low Drift
1
1
1
1
1
1
1
2
1
2
1
2
2
2
2
2
2
2
2
2
as 5 mA (60 dB greater than full scale) can be
tolerated at the input without overload.
The problem with this definition is the word
'tolerable. Clearly the noise at the dynamic reserve
limit should not cause an overload anywhere in the
instrument. This is accomplished by adjusting the
distribution of the gain. To achieve high reserve, the
input signal gain is set very low so the noise is not
likely to overload. This means that the signal at the
filter section is also very small. The filters then
remove the large noise components from the signal
which allows the remaining component to be
amplified to reach full scale. There is no problem
running the input amplifier at low gain. However,
large noise signals almost always disturb the
measurement in some way.
The most common effect of high dynamic reserve is
to generate noise and drift at the output. This comes
about because the output amplifier is running at very
high gain and front-end noise and offset drift will be
amplified and appear large at the output. The noise is
more tolerable than the DC drift errors since
increasing the time constant of the filters will
attenuate the noise.
Lastly, dynamic reserve depends on the noise
frequency. Clearly noise at the signal frequency will
make its way to the output without attenuation. So
the dynamic reserve at fsig is 0 dB. As the noise
frequency moves away from the signal frequency,
the dynamic reserve increases. Why? Because the
filters after the front-end attenuate the noise
components. The rate at which the reserve increases
depends upon the filter time constant and rolloff.
The reserve increases at the rate at which the filter
rolls off. When the noise frequency is far away, the
reserve is limited by the gain distribution and
overload level of each gain element.
In the SR570, decreasing front-end gain to increase
dynamic reserve can only be accomplished by
decreasing the value of the input op amp's feedback
resistor. Thus, the high dynamic reserve mode is also
a high bandwidth mode due to a smaller time
constant between the input capacitance and the
feedback resistor. On the other hand, a smaller
resistor means that even though the Johnson noise is
less, the extra gain of 10 at the output makes the
B-1
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