OP196/OP296/OP496

REV.

–12–

APPLICATIONS INFORMATION

Functional Description

The OP196 family of operational amplifiers is comprised of single-

supply, micropower, rail-to-rail input and output amplifiers. Input

offset voltage (VOS) is only 300

µV maximum, while the output

will deliver

±5 mA to a load. Supply current is only 50 µA, while

bandwidth is over 450 kHz and slew rate is 0.3 V/

µs. TPC 36 is

a simplified schematic of the OP196—it displays the novel cir-

cuit design techniques used to achieve this performance.

Input Overvoltage Protection

The OPx96 family of op amps uses a composite PNP/NPN

input stage. Transistor Q1 in Figure 36 has a collector-base

voltage of 0 V if +IN = VEE. If +IN then exceeds VEE, the junc-

tion will be forward biased and large diode currents will flow,

which may damage the device. The same situation applies to

+IN on the base of transistor Q5 being driven above VCC. There-

fore, the inverting and noninverting inputs must not be driven

above or below either supply rail unless the input current is

limited.

Figure 1 shows the input characteristics for the OPx96 family.

This photograph was generated with the power supply pins

connected to ground and a curve tracer’s collector output drive

connected to the input. As shown in the figure, when the input

voltage exceeds either supply by more than 0.6 V, internal

pn-junctions energize and permit current flow from the inputs

to the supplies. If the current is not limited, the amplifier may

be damaged. To prevent damage, the input current should be

limited to no more than 5 mA.

10

0%

100

90

8

6

4

2

0

–2

–4

–6

–8

–1.5

–1 –0.5

0

0.5

1

1.5

INPUT VOLTAGE – V

Figure 1. Input Overvoltage I-V Characteristics of the

OPx96 Family

Output Phase Reversal

Some other operational amplifiers designed for single-supply

operation exhibit an output voltage phase reversal when their

inputs are driven beyond their useful common-mode range.

Typically for single-supply bipolar op amps, the negative supply

determines the lower limit of their common-mode range. With

these common-mode limited devices, external clamping diodes

are required to prevent input signal excursions from exceeding

the device’s negative supply rail (i.e., GND) and triggering

output phase reversal.

The OPx96 family of op amps is free from output phase reversal

effects due to its novel input structure. Figure 2 illustrates the

performance of the OPx96 op amps when the input is driven

beyond the supply rails. As previously mentioned, amplifier

input current must be limited if the inputs are driven beyond

the supply rails. In the circuit of Figure 2, the source ampli-

tude is

± 15 V, while the supply voltage is only ± 5 V. In this

case, a 2 k

Ω source resistor limits the input current to 5 mA.

10

0%

100

90

VS = 5V

AV = 1

5V

1ms

5V

0

0

VIN

VOUT

TIME – 1ns/DIV

Figure 2. Output Voltage Phase Reversal Behavior

Input Offset Voltage Nulling

The OP196 provides two offset adjust terminals that can be

used to null the amplifier’s internal VOS. In general, operational

amplifier terminals should never be used to adjust system offset

voltages. A 100 k

Ω potentiometer, connected as shown in Fig-

ure 3, is recommended to null the OP196’s offset voltage. Offset

nulling does not adversely affect TCVOS performance, providing

that the trimming potentiometer temperature coefficient does

not exceed

±100 ppm/°C.

6

7

2

3

V–

V+

OP196

100k

4

1

5

Figure 3. Offset Nulling Circuit

Driving Capacitive Loads

OP196 family amplifiers are unconditionally stable with capaci-

tive loads less than 170 pF. When driving large capacitive loads

in unity-gain configurations, an in-the-loop compensation

technique is recommended, as illustrated in Figure 4.

OP296

CF

VIN

RG

RF

RX

CL

VOUT

RX =

WHERE RO = OPEN-LOOP OUTPUT RESISTANCE

RO RG

RF

CF = I + (

) (

) C

L RO

I

|ACL|

RF + RG

RF

Figure 4. In-the-Loop Compensation Technique for

Driving Capacitive Loads

E