Stanford Research Systems SR810 Manual page 57

and the frequency increases above 200 Hz, the time constant WILL
change to 30 s. Decreasing the frequency back below 200 Hz will NOT
change the time constant back to 100 s.
The absolute minimum time constant is 10 µs. The actual minimum time
constant depends upon the filter slope and the DC gain in the low pass
filter (dynamic reserve plus expand). The minimum time constant is only
restricted if the dynamic reserve plus expand is high and the filter slope
is low - not a normal operating situation. The tables below list the
minimum time constants for the different filter slopes and gains.
6 dB/oct DC gain (dB)
<45
<55
<65
<75
<85
<95
<105
<115
<125
<135
<145
<155
<165
<175
12 dB/oct DC gain (dB)
<55
<75
<95
<115
<135
<155
<175
18 dB/oct DC gain (dB)
<62
<92
<122
<152
<182
24 dB/oct DC gain (dB)
<72
<112
<152
<182
To use these tables, choose the correct table for the filter slope in use.
Calculate the DC gain by adding the reserve to the expand (expressed in
dB). Find the smallest DC gain entry which is larger than the gain in use.
Read the minimum time constant for this entry. For example, if the slope
is 12 dB/oct, the reserve is 64 dB, and the X expand is 10 (20 dB), then
the DC gain is 84 dB and the min time constant is 100 µs.
4-9
Front Panel
min time constant
10 µs
30 µs
100 µs
300 µs
1 ms
3 ms
10 ms
30 ms
100 ms
300 ms
1 s
3 s
10 s
30 s
min time constant
10 µs
30 µs
100 µs
300 µs
1 ms
3 ms
10 ms
min time constant
10 µs
30 µs
100 µs
300 µs
1 ms
min time constant
10 µs
30 µs
100 µs
300 µs