Datasheet OP777, OP727, OP747 (Analog Devices) - 12

OP777/OP727/OP747
and limiting device power dissipation is of prime importance in
15V
these designs. Figure 7 shows an example of 5 V, single-supply current monitor that can be incorporated into the design of a voltage
1k
regulator with foldback current limiting or a high current power
REF 2N2222
supply with crowbar protection. The design capitalizes on the
192 1/4 OP747
OP777’s common-mode range that extends to ground. Current
12k R2 4 3
is monitored in the power supply return where a 0.1 Ω shunt
20k +15V R1 R1
resistor, RSENSE, creates a very small voltage drop. The voltage at the inverting terminal becomes equal to the voltage at the noninverting
VO
terminal through the feedback of Q1, which is a 2N2222 or equiva-
R(1+ ) R +15V 1/4 OP747
lent NPN transistor. This makes the voltage drop across R1 equal to
15V R2
the voltage drop across R
VO = VREF
SENSE. Therefore, the current through Q1
R1 1/4 OP747
becomes directly proportional to the current through R
R
SENSE, and
= 15V R
the output voltage is given by: Figure 9. Linear Response Bridge ⎛ R2 ⎞ V = 5 V − × R × I A single-supply current source is shown in Figure 10 . Large resistors OUT SENSE L ⎝⎜ 1 ⎠⎟ R are used to maintain micropower operation. Output current can be adjusted by changing the R2B resistor. Compliance voltage is: The voltage drop across R2 increases with IL increasing, so VOUT decreases with higher supply current being sensed. For the element V ≤ V − V L SAT S values shown, the VOUT is 2.5 V for return current of 1 A.
10pF 3.0 V TO 30V 5V 100k R2 = 2.49k V 100k OUT OP777 Q1 R1 = 100k 5V R2B 2.7k 10pF IO R2 = R2A + R2B R2A + R1 = 100 OP777 R2 97.3k V R 0.1 IO = VS L LOAD RETURN TO R1 R2B GROUND RSENSE = 1mA 11mA
Figure 7. A Low-Side Load Current Monitor Figure 10. Single-Supply Current Source The OP777/OP727/OP747 is very useful in many bridge applica- A single-supply instrumentation amplifier using one OP727 tions. Figure 8 shows a single-supply bridge circuit in which its amplifier is shown in Figure 11. For true difference R3/R4 = output is linearly proportional to the fractional deviation () of R1/R2. The formula for the CMRR of the circuit at dc is CMRR = the bridge. Note that = ΔR/R. 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
= 300
percentage mismatch. We can rewrite the CMRR equation to
15V AR1 V V REF
reflect this CMRR = 20 × log (10000/% Mismatch). The key to
O = + 2.5V 2R2 2
high CMRR is a network of resistors that are well matched from
R1 1/4 OP747 = R1
the perspective of both resistive ratio and relative drift. It should
REF 6 RG = 10k 192
be noted that the absolute value of the resistors and their absolute
2 1M 2.5V 10.1k
drift are of no consequence. Matching is the key. CMRR is 100 dB
4 3 1M REF 0.1 F
with 0.1% mismatched resistor network. To maximize CMRR,
192 15V
one of the resistors such as R4 should be trimmed. Tighter match-
15V 4 3
ing of two op amps in one package (OP727) offers a significant
R1 R1(1+ ) 10.1k V1
boost in performance over the triple op amp configuration.
1/4 OP747 VO R1(1+ ) R1 1/4 OP747 R3 = 10.1k R2 = 1M R2 3.0 V TO 30V V2 3.0 V TO 30V R4 = 1M R1 = 10.1k
Figure 8. Linear Response Bridge, Single Supply
VO
In systems where dual supplies are available, the circuit of Figure
1/2 OP727 V1 1/2 OP727
9 could be used to detect bridge outputs that are linearly related
V2
to the fractional deviation of the bridge.
VO = 100 (V2 V1) 0.02mV V1 V2 290mV 2mV VOUT 29V USE MATCHED RESISTORS
Figure 11. Single-Supply Micropower Instrumentation Amplifier –12– REV. D Document Outline GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAMS SIMILAR LOW POWER PRODUCTS ELECTRICAL CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS Typical Performance Characteristics BASIC OPERATION OUTLINE DIMENSIONS ORDERING GUIDE REVISION HISTORY