Teledyne DA1855A, Operator'S Manual

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DA1855A Differential Amplifier
22 922258-00 Rev A
More sensitive settings (e.g. 100µV/div) available on some oscilloscopes can be used, but their
usefulness may be limited by noise, particularly with the DA1855A FULL bandwidth limit selection
and without averaging. With the oscilloscope set to 100µV/div and the DA1855A in the X10 GAIN
mode, the overall scale factor will be 10µV/div.
In the X10 GAIN mode, the DA1855A has lower noise than many oscilloscopes, so it is preferable to
use the /DA1855A X10 GAIN mode and a lower oscilloscope scale factor. For example, to obtain the
best noise performance at 1mV/div, set the DA1855A to X10 mode and the oscilloscope to 10mV/div
rather than the use X1 mode and 1mV/div. This also maximizes the bandwidth, as some
oscilloscopes give up some bandwidth at their most sensitive settings. Some oscilloscopes give up
bits of resolution to obtain 1mV or 2 mV/div sensitivity. The loss of resolution can be avoided by
using this technique. Any oscilloscope bandwidth limit setting may be used so long as the unlimited
signal does not exceed full screen before invoking bandwidth limit.
Probes and Differential Amplifiers
When using a differential amplifier it is very important to understand the role probes play in the
overall measurement system performance. Probes not only make attachment to the circuit under
test more convenient, ÷10 and ÷100 attenuating probes also extend the common mode range of the
differential amplifier. For example, the DA1855A amplifiers have a common mode range of ±15.5
volts when their internal attenuators are set to ÷1 and 155 volts when set to ÷10. The addition of a
probe with an attenuation factor of ten will extend the common mode range to 1550 volts or the
rating of the probe, whichever is less.
There is a trade-off, however. The Common Mode Rejection Ratio (CMRR) capability of even highly
matched differential probe pairs is seldom as good that of the amplifier. In order to preserve as
much of the amplifier’s performance as possible at the probe tips, it is important to use probes that
are designed for differential performance. Attempting to use normal ÷10 or ÷100 attenuating
oscilloscope probes, even high quality probes, will result in very poor CMRR performance. Nominally
matching ÷1 probes however, will provide excellent common mode rejection and are
recommended.
For applications which do not require additional attenuation, ÷1probes present relative high
capacitive loading to the circuit under test, limiting their usefulness to low frequency
measurements.
When making differential measurements, accurate probe compensation is much more important
than in single-ended measurements. Most probes depend on the accuracy of the oscilloscope’s 1
M
Ω
input resistor to determine the accuracy of the probe’s attenuation factor. Two probes with a
1% accuracy specification can yield a CMRR as low as 50 to 1 at DC while the amplifier CMRR may be
higher than 100,000 to 1. At high frequencies, the CMRR will be worse.
A differential probe pair must allow for matching at DC as well as over their useful frequency range.
Changing the compensation of a differentially matched probe set without following the proper