Line Frequency Rejection Filter; Ac Gain And Dynamic Reserve - Ametek 7230 Instruction Manual

Chapter 3, TECHNICAL DESCRIPTION

3.3.03 Line Frequency Rejection Filter

3.3.04 AC Gain and Dynamic Reserve

3-4
B (I) inputs when operating in differential input mode. It is usually given in decibels.
Hence a specification of > 100 dB implies that a common mode signal (i.e. a signal
simultaneously applied to both A and B (I) inputs) of 1 V will give rise to less than
10 µV of signal out of the input amplifier.
The input can also be set to the –B mode, in which case the lock-in amplifier
measures the voltage between the center and the shell of the B (I) input connector.
This extra mode effectively allows the input to be multiplexed between two different
single-ended signals, subject to the limitation that the user must allow for the signal
inversion (equivalent to a 180° phase-shift) which it introduces when reading the
outputs.
Input Connector Shell, Ground / Float
The input connector shells may be connected either directly to the instrument's
chassis ground or floated via a 1 k resistor. When in the float mode, the presence of
this resistor substantially reduces the problems that often occur in low-level lock-in
amplifier measurements due to ground loops.
Input Signal Selection, V / I
Although the voltage mode input is most commonly used, a current-to-voltage
converter may be switched into use to provide current mode input capability, in
which case the signal is connected to the B (I) connector. High impedance sources
(> 100 k) are inherently current sources and need to be measured with a low
impedance current mode input. Even when dealing with a voltage source in series
with a high impedance, the use of the current mode input may provide advantages in
terms of improved bandwidth and immunity from the effects of cable capacitance.
The converter may be set to low-noise or wide bandwidth conversion settings, but it
is worth noting that if the best possible performance is required a separate current
preamplifier, such as the SIGNAL RECOVERY models 181 or 5182, should be
considered.
Following the signal input amplifier there is an option to pass the signal through a
line frequency rejection filter, which is designed to give greater than 40 dB of
attenuation at the power line frequencies of 50 Hz or 60 Hz and their second
harmonics at 100 Hz and 120 Hz.
The filter uses two cascaded rejection stages with "notch" characteristics, allowing it
to be set to reject signals at frequencies equal to either of, or both of, the
fundamental and second harmonic of the line frequency.
The signal channel contains a number of analog filters and amplifiers whose overall
gain is defined by the AC Gain parameter, which is specified in terms of decibels
(dB). For each value of AC Gain there is a corresponding value of the INPUT LIMIT
parameter, which is the maximum instantaneous (peak) voltage or current that can be
applied to the input without causing input overload, as shown in table 3-1 below.