OP193/OP293

Rev. C | Page 14 of 20

INPUT OVERVOLTAGE PROTECTION

As previously mentioned, the OP193/OP293 op amps use a

PNP input stage with protection resistors in series with the

inverting and noninverting inputs. The high breakdown of the

PNP transistors, coupled with the protection resistors, provides

a large amount of input protection from overvoltage conditions.

The inputs can therefore be taken 20 V beyond either supply

without damaging the amplifier.

OUTPUT PHASE REVERSAL—OP193

The OP193’s input PNP collector-base junction can be forward-

biased if the inputs are brought more than one diode drop (0.7 V)

below ground. When this happens to the noninverting input,

Q4 of the cascode stage turns on and the output goes high. If

the positive input signal can go below ground, phase reversal

can be prevented by clamping the input to the negative supply

(that is, GND) with a diode. The reverse leakage of the diode

does add to the input bias current of the amplifier. If input bias

current is not critical, a 1N914 diode adds less than 10 nA of

leakage. However, its leakage current doubles for every 10°C

increase in ambient temperature. For critical applications, the

collector-base junction of a 2N3906 transistor adds only about

10 pA of additional bias current. To limit the current through

the diode under fault conditions, a 1 k Ω resistor isrecommended

in series with the input. (The OP193’s internal current limiting

resistors do not protect the external diode.)

OUTPUT PHASE REVERSAL—OP293

The OP293 includes two lateral PNP transistors, Q7 and Q8, to

protect against phase reversal. If an input is brought more than

one diode drop (≈0.7 V) below ground, Q7 and Q8 combine to

level shift the entire cascode stage, including the bias to Q3 and

Q4, simultaneously. In this case, Q4 does not saturate and the

output remains low.

The OP293 does not exhibit output phase reversal for inputs up

to −5 V below V− at +25°C. The phase reversal limit at +125°C

is about −3 V. If the inputs can be driven below these levels, an

external clamp diode, as discussed in the previous section,

should be added.

BATTERY-POWERED APPLICATIONS

OP193/OP293 series op amps can be operated on a minimum

supply voltage of 1.7 V, and draw only 13 μA of supply current

per amplifier from a 2.0 V supply. In many battery-powered cir-

cuits, OP193/OP293 devices can be continuously operated for

thousands of hours before requiring battery replacement, thus

reducing equipment downtime and operating cost.

High performance portable equipment and instruments fre-

quently use lithium cells because of their long shelf life, light

weight, and high energy density relative to older primary cells.

Most lithium cells have a nominal output voltage of 3 V and are

noted for a flat discharge characteristic. The low supply voltage

requirement of the OP193/OP293, combined with the flat

discharge characteristic of the lithium cell, indicates that the

OP193/OP293 can be operated over the entire useful life of the

cell. Figure 27 shows the typical discharge characteristic of a

1 Ah lithium cell powering the OP193 and OP293, with each

amplifier, in turn, driving 2.1 V into a 100 kΩ load.

2

1

HOURS

3

4

0

OP293

OP193

0

1000

2000

3000

4000

5000

6000

7000

Figure 27. Lithium Sulfur Dioxide Cell Discharge Characteristic with

OP193/OP293 and 100 kΩ Loads Input Offset Voltage Nulling

The OP193 provides two offset nulling terminals that can be

amplifier terminals should never be used to adjust system offset

voltages. The offset nulling circuit of Figure 28 provides about

±7 mV of offset adjustment range. A 100 kΩ resistor placed in

series with the wiper arm of the offset null potentiometer, as shown

in Figure 29, reduces the offset adjustment range to 400 μV and

is recommended for applications requiring high null resolution.

providing that the trimming potentiometer temperature coeffi-

cient does not exceed ±100 ppm/°C.

V+

OP193

100k

Ω

V–

2

3

1

5

6

4

7

Figure 28. Offset Nulling Circuit

V+

OP193

100k

Ω

100k

Ω

V–

2

3

1

5

6

4

7

Figure 29. High Resolution Offset Nulling Circuit