Datasheet

AD8546
Rev. A | Page 19 of 24
0
5
10
15
20
25
30
35
40
0 2 4 6 8 10 12 14 16 18
I
SY
PER DUAL AMPLIFIER (µA)
V
SY
(V)
I
SY
–
I
SY
+
09585-067
Figure 67. Supply Current vs. Supply Voltage (Voltage Follower)
In contrast to op amps, comparators are designed to work in an
open-loop configuration and to drive logic circuits. Although
op amps are different from comparators, occasionally an unused
section of a dual op amp is used as a comparator to save board
space and cost; however, this is not recommended.
Figure 68 and Figure 69 show the AD8546 configured as a com-
parator, with 100 kΩ resistors in series with the input pins. Any
unused channels are configured as buffers with the input voltage
kept at the midpoint of the power supplies.
AD8546
1/2
A1
100kΩ
100kΩ
I
SY
+
+V
SY
V
OUT
–V
SY
I
SY
–
A2
09585-068
Figure 68. Comparator A
AD8546
1/2
A1
100kΩ
100kΩ
I
SY
+
+V
SY
V
OUT
–V
SY
I
SY
–
A2
09585-069
Figure 69. Comparator B
The AD8546 has input devices that are protected from large differ-
ential input voltages by Diode D1 and Diode D2 (see Figure 62).
These diodes consist of substrate PNP bipolar transistors and
conduct whenever the differential input voltage exceeds approxi-
mately 600 mV; however, these diodes also allow a current path
from the input to the lower supply rail, thus resulting in an increase
in the total supply current of the system. As shown in Figure 70,
both configurations yield the same result. At 18 V of power supply,
I
SY
+ remains at 36 μA per dual amplifier, but I
SY
− increases to
140 μA in magnitude per dual amplifier.
0
20
40
60
80
100
120
140
160
0 2 4 6 8 10 12 14 16 18
I
SY
PER DUAL AMPLIFIER (µA)
V
SY
(V)
I
SY
–
I
SY
+
09585-070
Figure 70. Supply Current vs. Supply Voltage (AD8546 as a Comparator)
Note that 100 kΩ resistors are used in series with the input of
the op amp. If smaller resistor values are used, the supply current
of the system increases much more. For more information about
using op amps as comparators, see the AN-849 Application
Note, Using Op Amps as Comparators.
4 mA TO 20 mA PROCESS CONTROL CURRENT
LOOP TRANSMITTER
A 2-wire current transmitter is often used in distributed control
systems and process control applications to transmit analog signals
between sensors and process controllers. Figure 71 shows a 4 mA
to 20 mA current loop transmitter.
The transmitter is powered directly from the control loop power
supply, and the current in the loop carries signal from 4 mA to
20 mA. Thus, 4 mA establishes the baseline current budget
within which the circuit must operate.
The AD8546 is an excellent choice due to its low supply current
of 33 μA per amplifier over temperature and supply voltage. The
current transmitter controls the current flowing in the loop, where
a zero-scale input signal is represented by 4 mA of current and a
full-scale input signal is represented by 20 mA. The transmitter
also floats from the control loop power supply, V
DD
, whereas
signal ground is in the receiver. The loop current is measured
at the load resistor, R
L
, at the receiver side.