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參數資料
| 型號： | OP747AR |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 運動控制電子 |
| 英文描述： | Precision Micropower Single-Supply Operational Amplifiers |
| 中文描述： | QUAD OP-AMP, 300 uV OFFSET-MAX, 0.7 MHz BAND WIDTH, PDSO14 |
| 封裝： | SOIC-14 |
| 文件頁數： | 12/16頁 |
| 文件大小： | 194K |
| 代理商： | OP747AR |
REV. C
OP777/OP727/OP747
–
12
–
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
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:
shunt
V
V
R
R
R
I
OUT
SENSE
L
=
×
×
5
2
1
The voltage drop across
R
2 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.
5V
R2 = 2.49k
OP777
5V
R1 = 100
V
OUT
Q1
RETURN TO
GROUND
0.1
R
SENSE
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
=
R/R.
REF
192
15V
1M
R1(1+ )
R1
1/4 OP747
15V
15V
1M
1/4 OP747
V
O
10.1k
0.1 F
2.5V
1/4 OP747
R2
V2
V1
3
4
REF
192
2
2
10.1k
RG = 10k
R1(1+ )
R1
3
4
6
V
O
= AR1 V
+ 2.5V
R1
REF
=
R1
= 300
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.
REF
192
+15V
15V
V
O
= R2
REF
=
R
R
R2
R1
R
R1
+15V
15V
1/4 OP747
1/4 OP747
12k
15V
1k
V
O
3
2N2222
R(1+ )
1/4 OP747
20k
4
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
V
V
L
SAT
S
≤
I
O
= R1
S
R2B
R2
= 1mA 11mA
100k
OP777
R2A
97.3k
2.7V TO 30V
10pF
10pF
100k
R2B
2.7k
I
O
R
LOAD
+
V
L
R1 = 100k
R2 = R2A + R2B
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 the
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.
2.7V TO 30V
R2 = 1M
1/2 OP727
V
O
2.7V TO 30V
R3 = 10.1k
1/2 OP727
R1 = 10.1k
R4 = 1M
V1
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
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