B&K Precision 889B - Manual - Manual (Page 29)

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(b) BLOCK DIAGRAM (a) CONNECTION
DUT
V
A
(c) TYPICAL IMPEDANCE
MEASUREMENT RANGE(£[)
4T
1m 10m 100m 1 10 1K 10K 100K 1M 100 10M
H POT
DUT
H CUR
L CUR
L POT
H POT
DUT
H CUR
L CUR
L POT
(d) 4T CONNECTION WITH SHILDING
Figure 4.5
 Eliminating the Effect of the Parasitic Capacitor
When measuring the high impedance component (i.e. low capacitor), the parasitic capacitor becomes an
important issue (Figure 4.6). In figure 4.6(a), the parasitic capacitor Cd is paralleled to DUT as well as the Ci
and Ch. To correct this problem, add a guard plane (Figure 4.6(b)) in between H and L terminals to break the
Cd. If the guard plane is connected to instrument guard, the effect of Ci and Ch will be removed.
(a) Parastic Effect
H CUR H POT L POT L CUR
Cd
Connection
Point
DUT
C h C l
Ground
(b) Guard Plant reduces
Parastic Effect
H CUR H POT L POT L CUR
Guard
Plant
Figure 4.6
4.2 Open/Short Compensation
For those precision impedance-measuring instruments, the open and short compensation need to be used to
reduce the parasitic effect of the test fixture. The parasitic effect of the test fixture can be treated like the simple
passive components in figure 4.7(a). When the DUT is open, the instrument gets the conductance Yp = Gp +
j
ωCp (Figure 4.7(b)). When the DUT is short, the instrument gets the impedance Zs = Rs + jωLs (Figure 4.7(c)).
After the open and short compensation, the 889B has Yp and Zs that can then be used for the real Zdut
calculation (Figure 4.7(d)).