LeCroy DA1855A Operation - page 4
DA1855A Differential Amplifier
3-4
ISSUED: July 2002
DA1855A-OM-E Rev A
the oscilloscope screen. Additionally, the risk of tripping the input
overload detector and automatically disconnecting the input is
eliminated.
In the AC mode, the +INPUT is connected through an AC
coupling capacitor to the amplifier input or the input attenuator.
The coupling capacitor retains its charge when the input is
switched to DC, making it possible to return to the same circuit
without the precharge time. But this also makes it possible to
discharge the coupling capacitor into another circuit under test if
its DC voltage differs by more than approximately 19 V from the
voltage on the coupling capacitor.
Note:
The discharge current from the AC coupling capacitor is
limited to about 70 mA. In some situations this could
damage sensitive circuits. To avoid the inrush current
transient, it is therefore recommended that the +INPUT
coupling first be changed to the OFF (precharge) when
measuring a new circuit point. This will safely recharge
the AC coupling capacitor in less than 0.3 seconds.
DC and low frequencies are attenuated by the AC coupling
capacitor and the input resistance. With the ATTENUATOR set to
÷
÷10, or set to ÷
÷1 with the INPUT RESISTANCE set to 1 MΩ, the
low frequency cut off (-3dB point) is approximately 1.6 Hz. When
the input attenuator is set to
÷
÷1, the INPUT RESISTANCE may
be set to 100 M
Ω, and the –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 – V
COMP
)
The –INPUT has the same coupling modes as the +INPUT plus
one additional option, V
COMP
(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
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).