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OP293 Folha de dados(PDF) 11 Page - Analog Devices

Nome de Peças OP293
Descrição Electrónicos  Precision, Micropower Operational Amplifiers
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Fabricante Electrônico  AD [Analog Devices]
Página de início  http://www.analog.com
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OP293 Folha de dados(HTML) 11 Page - Analog Devices

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REV. B
OP193/OP293/OP493
–11–
6
5
7
4
1
2
3
V–
V+
OP193
100k
100k
Figure 5. High Resolution Offset Nulling Circuit
A Micropower False-Ground Generator
Some single-supply circuits work best when inputs are biased
above ground, typically at 1/2 of the supply voltage. In these
cases a false ground can be created by using a voltage divider
buffered by an amplifier. One such circuit is shown in Figure 6.
This circuit will generate a false-ground reference at 1/2 of the
supply voltage, while drawing only about 27
µA from a 5 V supply.
The circuit includes compensation to allow for a 1
µF bypass
capacitor at the false-ground output. The benefit of a large
capacitor is that not only does the false ground present a very low
dc resistance to the load, but its ac impedance is low as well. The
OP193 can both sink and source more than 5 mA, which improves
recovery time from transients in the load current.
6
7
2
3
10k
OP193
100
4
5V OR 12V
0.022 F
1 F
240k
240k
1 F
2.5V OR 6V
Figure 6. A Micropower False-Ground Generator
A Battery-Powered Voltage Reference
The circuit of Figure 7 is a battery-powered voltage reference
that draws only 17
µA of supply current. At this level, two AA
alkaline cells can power this reference for more than 18 months.
At an output voltage of 1.23 V @ 25
°C, drift of the reference is
only 5.5
µV/°C over the industrial temperature range. Load
regulation is 85
µV/mA with line regulation at 120 µV/V.
Design of the reference is based on the Brokaw bandgap core
technique. Scaling of resistors R1 and R2 produces unequal
currents in Q1 and Q2. The resulting
∆VBE across R3 creates a
temperature-proportional voltage (PTAT) which, in turn, pro-
duces a larger temperature-proportional voltage across R4 and
R5, V1. The temperature coefficient of V1 cancels (first order)
the complementary to absolute temperature (CTAT) coefficient
of VBE1. When adjusted to 1.23 V @ 25
°C, output voltage
tempco is at a minimum. Bandgap references can have start-up
problems. With no current in R1 and R2, the OP193 is beyond
its positive input range limit and has an undefined output state.
Shorting Pin 5 (an offset adjust pin) to ground forces the output
high under these circumstances and ensures reliable startup
without significantly degrading the OP193’s offset drift.
6
7
2
3
C1
1000pF
OP193
VBE2
4
R2
1.5M
Q1
VOUT
(1.23V @ 25°C)
5
R1
240k
V+
(2.5V TO 36V)
Q2
1
2
3
7
6
5
VBE1
MAT-01AH
VBE
R3 68k
R5 20k
OUTPUT
ADJUST
R4
130k
V1
Figure 7. A Battery-Powered Voltage Reference
A Single-Supply Current Monitor
Current monitoring essentially consists of amplifying the voltage
drop across a resistor placed in series with the current to be
measured. The difficulty is that only small voltage drops can be
tolerated, and with low precision op amps this greatly limits the
overall resolution. The single-supply current monitor of Figure
8 has a resolution of 10
µA and is capable of monitoring 30 mA
of current. This range can be adjusted by changing the current
sense resistor R1. When measuring total system current, it may
be necessary to include the supply current of the current moni-
tor, which bypasses the current sense resistor, in the final result.
This current can be measured and calibrated (together with the
residual offset) by adjustment of the offset trim potentiometer,
R2. This produces a deliberate temperature dependent offset.
However, the supply current of the OP193 is also proportional
to temperature, and the two effects tend to track. Current in R4
and R5, which also bypasses R1, can be adjusted via a gain trim.
6
7
2
3
OP193
4
R2
100k
VOUT =
100mV/mA(ITEST)
5
V+
1
R2
9.9k
R3
100k
R5
100
R1
1
TO CIRCUIT
UNDER TEST
ITEST
Figure 8. Single-Supply Current Monitor


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