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R6

5.1 k

Ω

Buffer

U1b

1/4 TLC274C

R1

240 k

Ω

C2

1

µF

R4

47 k

Ω

U1a

1/4 TLC274CN

U1c

1/4 TLC274CN

R2

10 k

Ω

R3

100 k

Ω

C1

0.1

µF

R10

100 k

Ω, 1%

R9

10 k

Ω, 1%

R7

1 M

Ω

R8

1.8 k

Ω, 1%

R5

1.8 k

Ω, 1%

DUT

Integrator

(X100)

C3

0.68

µF

C4

0.1

µF

Triangle

Generator

−

+

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.

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.

5

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