TLC374, TLC374Q, TLC374Y
LinCMOS QUADRUPLE DIFFERENTIAL COMPARATORS
SLCS118C – NOVEMBER 1983 – REVISED MARCH 1999
8
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
PARAMETER MEASUREMENT INFORMATION
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.
Voltage divider R9 and R10 provide 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.
+
–
DUT
+
–
–
+
U1b
1/4 TLC274CN
Buffer
C2
1 µF
R1
240 kΩ
U1a
1/4 TLC274CN
Triangle
Generator
R2
10 kΩ
R3
100 kΩ
C1
0.1 µF
R10
100 Ω, 1%
R9
10 kΩ, 1%
R8
1.8 kΩ, 1%
R7
1 MΩ
R6
5.1 kΩ
R5
1.8 kΩ, 1%
C3
0.68 µF
U1c
1/4 TLC274CN
Integrator
C4
0.1 µF
R4
47 kΩ
V
DD
V
IO
(X100)
Figure 2. Test Circuit for Input Offset Voltage Measurement
