TLC27M4, TLC27M4A, TLC27M4B, TLC27M4Y, TLC27M9

LinCMOS

™ PRECISION QUAD OPERATIONAL AMPLIFIERS

SLOS093C – OCTOBER 1987 – REVISED MAY 1999

18

POST OFFICE BOX 655303

PARAMETER MEASUREMENT INFORMATION

input bias current

Because of the high input impedance of the TLC27M4 and TLC27M9 operational amplifiers, attempts to

measure the input bias current can result in erroneous readings. The bias current at normal room ambient

temperature is typically less than 1 pA, a value that is easily exceeded by leakages on the test socket. Two

suggestions are offered to avoid erroneous measurements:

1.

Isolate the device from other potential leakage sources. Use a grounded shield around and between

the device inputs (see Figure 4). Leakages that would otherwise flow to the inputs are shunted away.

2.

Compensate for the leakage of the test socket by actually performing an input bias current test (using

a picoammeter) with no device in the test socket. The actual input bias current can then be calculated

by subtracting the open-socket leakage readings from the readings obtained with a device in the test

socket.

One word of caution—many automatic testers as well as some bench-top operational amplifier testers use the

servo-loop technique with a resistor in series with the device input to measure the input bias current; the voltage

drop across the series resistor is measured and the bias current is calculated. This method requires that a device

be inserted into the test socket to obtain a correct reading; therefore, an open-socket reading is not feasible

using this method.

V = VIC

14

8

1

7

Figure 4. Isolation Metal Around Device Inputs

(J and N packages)

low-level output voltage

To obtain low-supply-voltage operation, some compromise was necessary in the input stage. This compromise

results in the device low-level output being dependent on both the common-mode input voltage level as well

as the differential input voltage level. When attempting to correlate low-level output readings with those quoted

in the electrical specifications, these two conditions should be observed. If conditions other than these are to

be used, please refer to Figures 14 through 19 in the

Typical Characteristics of this data sheet.