SRS Labs SR560 manual Input Voltage Noise, Dynamic Reserve, Calibration And Repair

CALIBRATION AND REPAIR

INPUT VOLTAGE NOISE

The amplifier's input voltage noise approximates that of a 1000Ω resistor (about 4 nV/√Hz). For source impedances below 1000Ω, the output noise will be dominated by the amplifier's input voltage noise. A typical amplifier has an input voltage noise vs. frequency as shown in the figure below. Notice that the voltage noise rises at lower frequencies (“1/f" noise).

DYNAMIC RESERVE

The dynamic reserve of the amplifier is a measure of how large a signal can be present at the input to the amplifier without causing an overload condition.

The definition of dynamic reserve is:

•DR (dB) = 20 log (Vin(f) w/o overload / Vin for full scale)

Afull-scale output voltage is 10 Vpp. Signals at the output (or at any stage) which exceed 10 Vpp cause an overload. The dynamic reserve is greater than 0 dB only when the filters are used to remove unwanted signals.

The dynamic reserve is a function of frequency and depends on the amplifier

configuration (gain, filters and dynamic reserve setting). The figure below shows the dynamic reserve (and maximum input signal without overload) for a SR560 set to a gain of 1000, the high pass filter set to 1 kHz and the low pass filter set to 10 kHz (for a bandpass from 1 kHz to 10 kHz). The dynamic reserve characteristic is shown for both "High Dynamic Reserve" and "Low Noise" gain modes.

There are several features to note. In the bandpass region between 1 and 10 kHz the dynamic reserve is 0 dB. The dynamic reserve is 3 dB at the filter frequencies of 1 and 10 kHz. The dynamic reserve rises by

6 dB/oct (or 20 dB per decade) as the signal moves away from the pole frequency, since each RC filter attenuates the signal. If a faster roll-off for interfering signals were required, a 12 dB/octave HP or LP filter could be used.

The HIGH DR characteristic offers 16 dB more DR at low frequencies and 26 dB more at high frequencies. The high frequency DR is limited only by the maximum 3 Vpp limit of the input stage.

The maximum DR in the low noise mode is 36 dB. Since there is no gain between the HP and LP filters in the Low Noise gain mode, the DR is the same at very high frequencies and very low frequencies.

The input reference voltage noise for the High DR gain mode is about 10 nV/√Hz, compared to 4 nV/√Hz in the Low Noise gain mode. The table (middle of next page) summarizes the input referenced noise and maximum dynamic reserve for all gains.

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