TLC372-EP Datasheet, PDF (5/17 Pages) Texas Instruments – LinCMOS™ DUAL DIFFERENTIAL COMPARATORS

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TLC372-EP
LinCMOS™ DUAL DIFFERENTIAL COMPARATORS
SGLS385 – MARCH 2007
PARAMETER MEASUREMENT INFORMATION (continued)
Figure 2 illustrates a practical circuit for direct dc measurement of input offset voltage that does not bias the
comparator into the linear region. The circuit consists of a switching-mode servo loop in which U1a generates a
triangular waveform of approximately 20-mV amplitude. U1b acts as a buffer, with C2 and R4 removing any
residual dc offset. The signal is then applied to the inverting input of the comparator under test, while the
noninverting input is driven by the output of the integrator formed by U1c through the voltage divider formed by
R9 and R10. The loop reaches a stable operating point when the output of the comparator under test has a duty
cycle of exactly 50%, which can only occur when the incoming triangle wave is sliced symmetrically or when the
voltage at the noninverting input exactly equals the input offset voltage.
C3
V_DD
R5
0.68 µF
1.8 kΩ, 1%
U1b
1/4 TLC274C
+ ^Buffer
C2
1 µF
R6
5.1 kΩ
U1c
1/4 TLC274CN
C1
0.1 µF
−
R1
240 kΩ
R4
47 kΩ
U1a
1/4 TLC274CN
Triangle
Generator
R3
100 kΩ
R2
10 kΩ
DUT
R7
1 MΩ
R8
1.8 kΩ, 1%
Integrator
C4
0.1 µF
R10
100 kΩ, 1%
R9
10 kΩ, 1%
V_IO
(X100)
Figure 2. Circuit for Input Offset Voltage Measurement
Voltage divider R9 and R10 provides a step up of the input offset voltage by a factor of 100 to make
measurement easier. The values of R5, R8, R9, and R10 can significantly influence the accuracy of the reading;
therefore, it is suggested that their tolerance level be 1% or lower.
Measuring the extremely low values of input current requires isolation from all other sources of leakage current
and compensation for the leakage of the test socket and board. With a good picoammeter, the socket and board
leakage can be measured with no device in the socket. Subsequently, this open-socket leakage value can be
subtracted from the measurement obtained with a device in the socket to obtain the actual input current of the
device.
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