3.3
Precalibration Values
3 – 3
3.3.2
Calibration Words
Next, query the SIM980 over the remote interface to determine the
calibration word values. Send the remote queries:
OFST? 1
. . .
OFST? 6
and record the results.
3.3.3
DC Gain Match
Set the DC voltage source to 2 V. Connect the BNC-Tee adapters
together to form a 1-to-4 splitter. Connect the splitter to the DC
voltage source, and connect the four SIM980 inputs to the splitter
with equal-length cables (important for AC testing, later). Enable
all four input channels, and set all input polarities to
+
. Note the
output voltage as
V
+
1
. Then change all four input polarities to
−
,
and note the output voltage as
V
−
1
.
Now switch the DC voltage source to
−
2 V, and with the channel
polarities still set to
−
note the output voltage as
V
−
2
. Change all
four input polarities to
+
, and finally note the output voltage as
V
+
2
.
Calculate the gain ratio mismatch as
V
+
1
−
V
+
2
V
−
1
−
V
−
2
−
1
!
×
100 %
and record this value as the “DC mismatch”.
3.3.4
AC Gain Match
Using the same 1-to-4 splitter from section 3.3.3, connect all four
input channels to the sine generator, set to create a 1 MHz sine wave
at 1 Vrms or 1 Vpeak-peak amplitude (circle which). Unlike the DC
case, here it is important that all four BNC cables leading to the
channel inputs are of exactly the same length and type and that
the splitting of the signal is accomplished in a symmetric manner.
Di
ff
erences in cable length will introduce phase shifts between inputs
and falsify the result.
Connect the output of the SIM980 to the oscilloscope. Separately
trigger the scope synchronously with the sine generator.
AC matching is measured pairwise between channels, with the two
channels under test set to opposite polarity and the remaining two
channels switched o
ff
. There are 12 such permutations, which can
be labeled with a
+
−
notation. For example, setting Channel 1
SIM980
Analog Summing Amplifier