OP193 Datasheet, PDF (10/16 Pages) Analog Devices – Precision, Micropower Operational Amplifiers

OP193/OP293/OP493
Driving Capacitive Loads
OP193 family amplifiers are unconditionally stable with capaci-
tive loads less than 200 pF. However, the small signal, unity-
gain overshoot will improve if a resistive load is added. For
example, transient overshoot is 20% when driving a 1000 pF/
10 kΩ load. When driving large capacitive loads in unity-gain
configurations, an in-the-loop compensation technique is rec-
ommended as illustrated in Figure 28.
Input Overvoltage Protection
As previously mentioned, the OP193 family of 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 over voltage 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 (i.e., GND) with a diode. The reverse leakage of the
diode will, of course, add to the input bias current of the ampli-
fier. If input bias current is not critical, a 1N914 will add less
than 10 nA of leakage. However, its leakage current will double
for every 10°C increase in ambient temperature. For critical
applications, the collector-base junction of a 2N3906 transistor
will only add about 10 pA of additional bias current. To limit
the current through the diode under fault conditions, a 1 kΩ
resistor is recommended in series with the input. (The OP193’s
internal current limiting resistors will not protect the external
diode).
Output Phase Reversal—OP293 and OP493
The OP293 and OP493 include 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 com-
bine to level shift the entire cascode stage, including the bias to
Q3 and Q4, simultaneously. In this case Q4 will not saturate
and the output remains low.
The OP293 and OP493 do 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 sec-
tion, should be added.
Battery Powered Applications
OP193 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 devices can be continuously operated for thou-
sands 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, combined with the flat discharge
characteristic of the lithium cell, indicates that the OP193 can
be operated over the entire useful life of the cell. Figure 25
shows the typical discharge characteristic of a 1 AH lithium cell
powering the OP193, OP293, and OP493, with each amplifier,
in turn, driving 2.1 Volts into a 100 kΩ load.
4
3
2
OP493
OP293
1
OP193
0
0 1000 2000 3000 4000 5000 6000 7000
HOURS
Figure 25. Lithium Sulfur Dioxide Cell Discharge Charac-
teristic with OP193 Family and 100 kΩ Loads
Input Offset Voltage Nulling
The OP193 provides two offset nulling terminals that can be
used to adjust the OP193’s internal V_OS. In general, operational
amplifier terminals should never be used to adjust system offset
voltages. The offset null circuit of Figure 26 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 27, reduces the offset adjustment range to
400 µV and is recommended for applications requiring high null
resolution. Offset nulling does not adversely affect TCV_OS per-
formance, providing that the trimming potentiometer tempera-
ture coefficient does not exceed ± 100 ppm/°C.
V+
7
2
OP193 6
4
3
5
1
100kΩ
V–
Figure 26. Offset Nulling Circuit
–10–
REV. A