TLC274, TLC274A, TLC274B, TLC274Y, TLC279

LinCMOS

™ PRECISION QUAD OPERATIONAL AMPLIFIERS

SLOS092C – SEPTEMBER 1987 – REVISED MARCH 1998

18

POST OFFICE BOX 655303

PARAMETER MEASUREMENT INFORMATION

input bias current

Because of the high input impedance of the TLC274 and TLC279 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.

input offset voltage temperature coefficient

Erroneous readings often result from attempts to measure temperature coefficient of input offset voltage. This

parameter is actually a calculation using input offset voltage measurements obtained at two different

temperatures. When one (or both) of the temperatures is below freezing, moisture can collect on both the device

and the test socket. This moisture results in leakage and contact resistance, which can cause erroneous input

offset voltage readings. The isolation techniques previously mentioned have no effect on the leakage since the

moisture also covers the isolation metal itself, thereby rendering it useless. It is suggested that these

measurements be performed at temperatures above freezing to minimize error.