IET Labs, Inc. RLC Digibridge 1693, User And Service Manual, Page: 85 / 171

61
1693 RLC Digibridge
Operation
NOTE: The BIAS ON indicator serves to indicate
that the EXTERNAL BIAS is switched ON, NOT
NECESSARILY the presence of external bias.
When the EXTERNAL BIAS switch is ON, the
BIAS ON indicator is lit as long as the Digibridge
POWER is ON. (The indicator brightness depends
somewhat on the external bias voltage.) Also , when
the EXTERNAL BIAS switch is ON, but the POWER
is switched OFF, this indicator is lit by external bias
voltages above about 3 V.
POWER ON. This is generally necessary to permit
the power-up self checks to pass.
Protection.
The Digibridge is internally protected
from damage from charged capacitors with stored
energy up to 1 joule at any voltage up to 60 V.
Operating Procedure.
1. If TRIGGERED measure mode is to be
used, calculate the delay that is suitable for
the largest value capacitor in the group to be
measured with external bias,thus:
Delay = (Cx Vbias) / Imax + 10 Rstd Cx seconds
NOTE: Cx is the capacitance of the DUT in farads.
Vbias is the external bias voltage in volts. Imax is the
maximum current from the external supply (usually
0.2) amperes. 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.) If the calculated delay is greater than
the normal settling time (7 to 12 ms for I-kHz mea-
surements), then program the Digibridge to use this
delay. See paragraph 3.5.3.
CAUTION
If your test procedure includes
charging capacitors to higher energy
or higher voltage before or during
connect ion to the Digibridg e ,
EXTERNAL PRECAUTIONS MUST
B E TA K E N T O P R O T E C T T H E
INSTRUMENT
2. To enable the special shorting routine (see
paragraph 3.7.3):
A. Select ENTER function
B. Press [2][=][SHIFT][SPECIAL][3]
3. Observe correct polarity when inserting DUT
into test fixture.
Bias POSITIVE polarity is at the LEFT (“low”
terminals) of the remote test fixture 1689-9600.
Bias NEGATIVE polarity is at the RIGHT.
4. Use the external switches (user-supplied)
to remove bias from the test fixture (recom-
mended when using external bias):
A. Apply bias after the DUT is in place
B. Remove bias after measurement
C. Short the DUT before its removal
However, for occasional (non-production)
measurements, if the capacitances being measured are
less than 200 µF and the bias voltage less than 30 V, an
optional procedure is to leave the external bias circuitry
“on” during measurements and to use the Digibridge
EXTERNAL BIAS switch to apply bias to the DUT (ON)
and to remove it and discharge the DUT (OFF).
5. For each DUT, in the CONTINUOUS mea-
sure mode, disregard the first displayed result
and read the second. Notice enough of the
subsequent results to verify that the DUT has
stabilized. Use the stable result.
6. In the TRIGGERED measure mode, each
measurement cycle includes the normal set-
tling time (7 to 12 ms for 1-kHz measure-
ments), or a programmed delay. Remeasure
enough DUTs to be sure that they are sta-
bilized in the first measurement so that any
subsequent differences are well within the
error permitted by your needs. If not, program
in a longer delay.
NOTE: There are two effects to be aware of in watch-
ing for stabilization of he 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 capaci-
tors respond slowly to a change in applied voltage,
therefore the DUT capacitance can be settling long
after the voltage is essentially stable.