OP193FSZ Datasheet, PDF (14/20 Pages) Analog Devices – Precision, Micropower Operational Amplifiers

OP193/OP293
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.
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.
4
3
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 irsecommended
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.
2
OP293
OP193
1
0
0
1000 2000 3000 4000 5000 6000 7000
HOURS
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
used to adjust the OP193’s internal VOS. In general, operational
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.
Offset nulling does not adversely affect TCVOS performance,
providing that the trimming potentiometer temperature coeffi-
cient does not exceed ±100 ppm/°C.
V+
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.
2
7
OP193
6
4
3
5
1
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
100kΩ
V–
Figure 28. Offset Nulling Circuit
V+
2
7
OP193
6
4
3
5
1
100kΩ
100kΩ
V–
Figure 29. High Resolution Offset Nulling Circuit
Rev. C | Page 14 of 20