OP747ARZ Datasheet, PDF (12/16 Pages) Analog Devices – Precision Micropower Single-Supply Operational Amplifiers

OP777/OP727/OP747
and limiting device power dissipation is of prime importance in
these designs. Figure 7 shows an example of 5 V, single-supply
current monitor that can be incorporated into the design of a voltage
regulator with foldback current limiting or a high current power
supply with crowbar protection. The design capitalizes on the
OP777’s common-mode range that extends to ground. Current
is monitored in the power supply return where a 0.1 Ω shunt
resistor, R_SENSE, creates a very small voltage drop. The voltage at the
inverting terminal becomes equal to the voltage at the noninverting
terminal through the feedback of Q1, which is a 2N2222 or equiva-
lent NPN transistor. This makes the voltage drop across R1 equal to
the voltage drop across R_SENSE. Therefore, the current through Q1
becomes directly proportional to the current through R_SENSE, and
the output voltage is given by:
V_OUT
= 5V
−
⎛
⎝⎜
R2
R1
×
R_SENSE
×
I
L
⎞
⎠⎟
The voltage drop across R2 increases with I_L increasing, so V_OUT
decreases with higher supply current being sensed. For the element
values shown, the V_OUT is 2.5 V for return current of 1 A.
V_OUT
5V
R2 = 2.49k⍀
Q1
5V
R1 = 100⍀
0.1⍀
R_SENSE
OP777
RETURN TO
GROUND
Figure 7. A Low-Side Load Current Monitor
The OP777/OP727/OP747 is very useful in many bridge applica-
tions. Figure 8 shows a single-supply bridge circuit in which its
output is linearly proportional to the fractional deviation (␦) of
the bridge. Note that ␦ = ΔR/R.
15V
1/4 OP747
2
REF
192
2.5V
0.1␮F
43
R1
R1(1+␦)
R2
2
REF ⁶
192
1M⍀
43
= 300
V_O
=
AR1؋V_REF
2R2
␦ + 2.5V
␦ = ^⌬R1
RG = 10k⍀
R1
10.1k⍀
1M⍀
R1(1+␦)
V1
R1
15V
10.1k⍀
1/4 OP747
15V
V_O
1/4 OP747
V2
Figure 8. Linear Response Bridge, Single Supply
In systems where dual supplies are available, the circuit of Figure
9 could be used to detect bridge outputs that are linearly related
to the fractional deviation of the bridge.
15V
REF
192
43
1k⍀
1/4 OP747
12k⍀
2N2222
20k⍀
R1
R1
R2
+15V
+15V ^R(1+␦)
R
1/4 OP747
؊15V
V_O
1/4 OP747
؊15V
V_O =
R2
R1
V_REF
␦
␦ = ^⌬R
R
Figure 9. Linear Response Bridge
A single-supply current source is shown in Figure 10 . Large resistors
are used to maintain micropower operation. Output current can be
adjusted by changing the R2B resistor. Compliance voltage is:
V_L ≤ V_SAT − V_S
3.0 V TO 30V
10pF
100k⍀
R1 = 100k⍀
10pF
R2 = R2A + R2B
I_O =
R2
R1 ؋ R2B
V_S
= 1mA ؊ 11mA
100k⍀
OP777
R2A
97.3k⍀
R2B
2.7k⍀
I_O
+
V_L R_LOAD
؊
Figure 10. Single-Supply Current Source
A single-supply instrumentation amplifier using one OP727
amplifier is shown in Figure 11. For true difference R3/R4 =
R1/R2. The formula for the CMRR of the circuit at dc is CMRR =
20 × log (100/(1–(R2 × R3)/(R1× R4)). It is common to specify t he
accuracy of the resistor network in terms of resistor-to-resistor
percentage mismatch. We can rewrite the CMRR equation to
reflect this CMRR = 20 × log (10000/% Mismatch). The key to
high CMRR is a network of resistors that are well matched from
the perspective of both resistive ratio and relative drift. It should
be noted that the absolute value of the resistors and their absolute
drift are of no consequence. Matching is the key. CMRR is 100 dB
with 0.1% mismatched resistor network. To maximize CMRR,
one of the resistors such as R4 should be trimmed. Tighter match-
ing of two op amps in one package (OP727) offers a significant
boost in performance over the triple op amp configuration.
R3 = 10.1k⍀
R2 = 1M⍀
3.0 V TO 30V
R4 = 1M⍀
R1 = 10.1k⍀
3.0 V TO 30V
1/2 OP727
V1
V_O
1/2 OP727
V2
V_O = 100 (V2 ؊ V1)
0.02mV V1 ؊ V2 290mV
2mV V_OUT 29V
USE MATCHED RESISTORS
Figure 11. Single-Supply Micropower Instrumentation
Amplifier
–12–
REV. ^D