Teledyne LeCroy DA1855A - Manual (Page 20)

DA1855A Differential Amplifier
14 922258-00 Rev A
–3 dB point is 0.016 Hz. This extremely low frequency
cut off is useful for observing low frequency noise riding on larger DC voltages.
In the DC mode, the +INPUT connector is connected to the amplifier either directly or through the
input attenuator, and the AC and DC attenuation are the same.
- Input Coupling (AC–OFF – DC – VCOMP)
The –INPUT has the same coupling modes as the +INPUT plus one additional option, VCOMP
(comparison voltage).
The DA1855A contains a precision DC voltage source which is controlled by the oscilloscope OFFSET
control. (When the amplifier is used stand alone, without ProBus interface to a Teledyne LeCroy
oscilloscope, the voltage is controlled by the push buttons above and below the front panel
numerical display.) This voltage source is called the Precision Voltage Generator (PVG).
The DA1855A's amplifier subtracts the voltage applied to its inverting input from the voltage applied
to its non-inverting input. The DA1855A output is therefore zero whenever these two voltages are
equal. For this reason, the voltage applied to the inverting input is called a comparison voltage,
VCOMP. Stated another way, the value of the horizontal center line in the oscilloscope graticule is
the voltage read in the PVG display. Each graticule line above or below the center line will add or
subtract the Volts/div value from the PVG setting. Refer to Error! Reference source not found.
where the horizontal center line represents a power supply voltage of 5.030 V, the next higher line
5.050 V and the line below the center line 5.010 V. In this figure noise on a + 5.030 V signal is easily
displayed using 5.030 Volt offset and a vertical scale factor of 20 mV/div.
VCOMP can be used to make precise measurements of large signals by comparing the accurately
known VCOMP with the unknown signal. It can also be used to measure the actual voltage at any
point of a waveform.
Since the amplifier’s gain and input attenuator are individually selectable, the comparison range can
be changed from ± 15.500 V to ± 155.000 V by changing the ATTENUATION from ÷1 to ÷10, while
the overall gain can still be set either to 1 or 0.1 by selecting either X10 or X1 GAIN.
NOTE: While in V
COMP
mode, the amplifier is configured for single ended measurements. The –INPUT
connector is not usable when V
COMP
is selected. The input signal applied to the + INPUT is referenced
to ground offset by the value set by the Precision Voltage Generator. Large calibrated offsets can be
obtained while making differential measurements by using V
DIFF
mode.
Precision Voltage Generator
The PVG generates the voltage which is used in the V
COMP
and V
DIFF
modes and appears at the rear
panel OFFSET VOLTAGE (PVG) output connector for use as a reference voltage.
The Precision Voltage Generator (PVG) output range is ± 15.500 Volt. The PVG is never attenuated
by the input attenuator. Attenuation of the +INPUT signal by the ÷10 input attenuator will cause the