Low-voltage limiter circuit suppresses the output of platinum probes at temperatures beyond their operating range. The limiter circuit comprises an operational amplifier with a dual feedback loop. The signal limiter is useful in low-voltage instrumentation circuits normally operable or set for cryogenic temperatures

Hansen, I. G.

Peterson, V. S.

English

NASA Technical Reports Server

February 1969	 Brief 69-10015 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Millivolt Signal Limiter 
D1 IN 914A 
R4	
Amplifier 
+	 R1	 Output
	 is 
Signal IN 2202
	 lOOK
	 Limited Signal Out 
R2	 Common	 - 
Although amplifiers and limiters have been in use 
for a considerable period of time, they exhibit char-
acteristics that preclude their use as low-level limiters; 
e.g., 12 millivolts or less. Offset current and/or volt-
age at the input of the amplifier add directly to the 
signal, thereby causing an error. Furthermore, all cur-
rent circuitry passes the signal through the amplifier 
with the resultant effects of zero drift, gain drift, and 
nonlinearities which distort the signal level. With pla-
tinum probes set for cryogenic tests, the output at 
room temperature is high enough to cause data ac-
quisition system malfunction, crosstalk, or break-
down. 
A low-voltage limiter circuit (under 12 millivolts) 
has been developed which suppresses the output of 
platinum probes at temperatures beyond their oper-
ating range. The limiter circuit comprises an opera-
tional amplifier with a dual feedback loop. For opera-
tion with the input voltage in the normal range, the 
negative feedback loop is completed through R4 to the
summing point at the amplifier input. Since the ampli-
fier open loop gain is high, the voltage at the ampli-
fier's input is held at zero. This in effect places R3 
in shunt across the output. The shunt effect is insig-
nificant because R 3 is much higher than R 1 and R2. 
For operation with the input voltage beyond the 
normal range, an additional negative feedback loop 
is completed through D 1 to the summing point at the 
junction of R 1 and R 3 . Since this point is also the 
output of the limiter, it is held by negative feedback 
to a level which, when amplified and fed back, will 
allow the diode to conduct. The summing junction at 
R 1 and R 3 in effect places a variable resistance shunt 
across the limiter output so as to produce a voltage 
drop across R 1 and R2 and maintain the limiting 
level. This signal limiting level can be varied by chang-
ing the closed loop gain of the amplifier or the initial 
offset level of the amplifier. Drift and gain variation 
affect only the limiting point; they have no effect on 
the signal over its normal operating range.
I cent in tied oserleall 
This document was prepared under the sponsorship of the National	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States	 information contained in this document, or warrants that such use 
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This signal limiter is useful in low-voltage instru-
mentation circuits normally operable or set for cryo-
genic temperatures. The limiter is also applicable to 
cryogenic instrumentation where there is danger that 
voltage output at room temperature might cause read-
out circuit failures. 
Notes: 
I. No additional documentation is available. 
2. Technical questions concerning this invention may 
be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B69-10015
Patent status: 
This invention has been patented by NASA (U.S. 
Patent No. 3,404,348), and royalty-free license rights 
will be granted for its commercial development. In-
quiries about obtaining a license should be addressed 
to NASA, Code GP, Washington, D.C. 20546. 
Source: Irving G. Hansen and Victor S. Peterson 
(LEW-90297) 
Brief 69-10015	 Category 01


Millivolt signal limiter

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