circuits鈥?performance in the process. It鈥檚 an especially tricky
erties. In such an application (see Figure 1), an ultra-low
measure a dielectric鈥檚 response to a 100V step.
shorts. Resistor R
to the 50mA level specified in the FET鈥檚 data sheet. R
2k鈩?value.
鈭?/div>
idt
OPA
100
90
80
70
60
Thermal and
Noise Threshold
10
0
5
10
V
DG
(V)
15
20
25
STANDARD DIELECTRIC-EVALUATION SCHEMES can prove disas-
trous to the op amp they employ. If the dielectric under test shorts, the
resulting high voltage at the op amp鈥檚 input destroys the device unless the
device鈥檚 very-low-bias (low voltage) input stage is somehow protected. The
solution is difficult: Whatever you do, it must not degrade the op amp鈥檚
performance.
FIGURE 1. Dielectric Evaluation Circuit.
Unfortunately, the op amp is destroyed if the dielectric
sample shorts.
For one such measurement setup, low-bias current op amps
like the OPA111, OPA121, OPA128, OPA124 or OPA129
can serve because their bias current is in the pA or even fA
range and therefore contributes negligible measurement er-
ror. What type of protective device doesn鈥檛 degrade this op
amp鈥檚 parameters? PN-junction devices usually have leak-
age currents in the nanoamp range even at very-low bias
voltages鈥攁 degradation of several orders of magnitude.
FETs are generally much better in this respect, and Siliconix鈥檚
2N4117A JFET proves the best.
Figure 2 shows an experimentally derived curve of leakage
current vs voltage for this device. Note that for voltages
comparable to those between an op amp鈥檚 inputs, the
2N4117A鈥檚 leakage is compatible with the op amp鈥檚 bias.
(The residual 60fA level at 0V arises from thermal effects
and measurement-system noise.)
The overload-protected design resulting from these FET
measurements is shown in Figure 3. The diode-connected
JFET serves as a shunt across the op amp鈥檚 input鈥攁 scheme
JFET LEAKAGE CURRENTS can be so low that they don't degrade a low-
input bias op amp's parameters. Measurements show that Siliconix's 2N4117A
can serve as a voltage limiter when it's diode-connected. The actual leakage
is probably somewhat lower than measured, but thermal effects and noise
place a limiting threshold on the measurement.
FIGURE 2. Curve of Leakage Current vs Voltage.
R
1
2k鈩?/div>
G
2N4117A
Output
Step
D
S
OPA
OVERVOLTAGE PROTECTION results when you incorporate a diode-
connected JFET in the measuring circuit. If the dielectric shorts during
testing, the FET clamps at 0.6V saving the sensitive op amp from destruc-
tion. R
1
's resistance鈥攁lthough high enough to protect the FET against
overcurrent failure鈥攊s still so small relative to the dielectric's impedance that
it doesn't impair measurement accuracy.
FIGURE 3. Overvoltage Protection Circuit.
Reprinted from EDN, October 5, 1980;
漏1980, Cahners Publishing Company.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
no responsibility for the use of this information, and all use of such information shall be entirely at the user鈥檚 own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.
漏
1994 Burr-Brown Corporation
AB-064
1
Printed in U.S.A. January, 1994
Application Bulletin Number 64
1
