(as is the case here) the noise picked up by the shield will also appear on the center conductor. This is good, because the lock-in's 100 dB CMRR will reject most of this common mode noise. However, not all of the noise can be rejected, especially the high frequency noise, and so the lock-in may overload on the high sensitivity ranges.

Quasi-Differential Connection

The second method of connecting the experiment to the lock-in is called the 'true-differential' mode. Here, the lock-in uses the difference between the center conductors of the A & B inputs as the input signal. Both of the signal sources are shielded from spurious pick-up.

With either method, it is important to minimize both the common mode noise and the common mode signal. Notice that the signal source is held near ground potential in both cases. A signal which appears on both the A & B inputs will not be perfectly cancelled: the common mode rejection ratio (CMRR) specifies the degree of cancellation. For low frequencies the CMRR of 100 dB indicates that the common mode signal is canceled to 1 part

in 105, but the CMRR decreases by about 6 dB/octave (20 dB/Decade) starting at 1KHz. Even

with a CMRR of 105, a 10 mV common mode signal behaves like 100nV differential signal.

True-Differential Connection

There are some additional considerations in deciding how to operate the lock-in amplifier:

Dynamic Reserve (DR) is the ratio of the largest noise signal that the lock-in can tolerate before overload to the full scale input. Dynamic reserve is usually expressed in dB. Thus a DR of 60 dB means that a noise source 1000 times larger than a full scale input can be present at the input without affecting the measurement of the signal. A higher DR results in a degraded output stability since most of the gain is DC gain after the phase sensitive detector. In general, the lowest DR which does not cause an overload should be used.

The Current Input has a 1 kinput impedance

and a current gain of 106 Volts/Amp. Currents from 500 nA down to 100 fA full scale can be measured. The impedance of the signal source is the most important factor to consider in deciding between voltage and current measurements.

For high source impedances, (>1 M) or small currents, use the current input. Its relatively low impedance greatly reduces the amplitude and phase errors caused by the cable capacitance- source impedance time constant. The cable capacitance should still be kept small to minimize the high frequency noise gain of the current preamplifier.

For moderate source impedances or larger currents, the voltage input is preferred. A small value resistor may be used to shunt the source. The lock-in then measures the voltage across this resistor. Select the resistor value to keep the source bias voltage small while providing enough signal for the lock-in to measure.

The Auto-Tracking Bandpass Filter has a Q of 5 and follows the reference frequency. The passband is therefore 1/5 of the reference frequency. The bandpass filter can provide an additional 20 dB of dynamic reserve for noise signals at frequencies outside the passband. The filter also improves the harmonic rejection of the lock-in. The second harmonic is attenuated an additional 13dB and higher harmonics are attenuated by 6 dB/octave more. You may wish to use the bandpass filter and select a low dynamic reserve setting in order to achieve a better output stability. Since the processor can only set the bandpass filter's center frequency to within 1% of the reference frequency, this filter can contribute up to 5° of phase shift error and up to 5% of amplitude error when it is used. In addition, the

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