External Bias - IET Labs, Inc. RLC Digibridge 1693 User And Service Manual

1693 RLC Digibridge
NOTE: There are two effects to be aware of in watch-
ing for stabilization of the DUT: voltage and capaci-
tance. Besides charging to a "final" voltage, there is
also the stabilization of capacitance value itself. For
example, some aluminum electrolytic capacitors re-
spond slowly to a change in applied voltage, therefore
the DUT capacitance can be settling long after the
voltage is essentially stable.
Normally, the delay for internal bias measurements
should be about 0 Rstd Cx seconds
(If the internal bias is being switched off during each
change of DUT - by remote control perhaps - this
delay should be 1 second larger: 1 + 10 Rstd Cx.)
NOTE: Rstd is 102400 for range 1, 6400 for range
2, 400 for range 3, 25 for range 4. (see Table 3-7 on
page 43.) Cx is the capacitance of the DUT in farads:
For example, measuring 2000 pF at 1 kHz (range
1), this delay time should be about (10)(102 400)
(.000 000 002) = approx .002 seconds. (Normal set-
tling time is adequate.)
7. After biased measurements are completed,
remember to disable the shorting routine:
A. Select the ENTER function
B. Press [0][=][SHIFT][SPECIAL][3]
8. Remove internal bias by pressing the
[SHIFT][INT BIAS] keys, so that the BIAS
ON indicator is NOT lit.
NOTE: The BIAS ON indicator serves to indicate
whether internal bias is connected or disconnected
only if the EXTERNAL BIAS is switched OFF. (See
below for external bias.)
Notice that repeating the same keyboard sequence
will cyclically enable and disable internal bias. For
best results, after removing bias and before making
further measurements, allow least 2 seconds for in-
ternal circuit discharge and settling.
60

3.7.2 External Bias

If bias is required at some other voltage than the 2-V
internal bias , use external bias as described below.
Also:
• Be sure that the voltage is never more than
60 V, max.
• A current-limiting power supply is recom-
mended; set the current limit at 200 mA.
• Be sure that the bias supply is floating; DO
NOT connect either lead to ground.
• Generally the external circuit must include
switching for application of bias after each
DUT is in the test fixture, and discharge before
it is removed.
• A well-filtered power supply is recommended.
Bias-supply hum can affect some measure-
ments, particularly if a test frequency is the
same as the power frequency.
Setup Procedure.
1. Connect the external bias voltage sup-
ply and switching circuit to the rear-panel
EXTERNAL BIAS connector using a pur-
chased or fabricated cable assembly. Observe
polarity marking on the rear panel.
2. Set the external supply to limit current
(<200 mA).
3. Set the external bias supply to the desired
voltage (<60 V).
4. If the Digibridge power is off, switch its
POWER ON, and wait for completion of the
self-check routine before the next step.
5. Switch the EXTERNAL BIAS ON (switch is
at right of keyboard) and verify that the BIAS
ON indicator is lit - see below.
If polarity is inverted, the indicator will not be lit as
brightly as normal.
6. If the bias cable fuse must be replaced, use a
200 mA fast-acting fuse.
7. Switch the bias off using an external switch,
so that the DUT can be inserted before
bias is applied to it. Refer to the Operating
Procedure below.
Operation