AD8546 datasheet(19/24 Pages) AD

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AD8546

Rev. 0 | Page 19 of 24

VSY (V)

ISY–

ISY+

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

100kΩ

ISY+

+VSY

VOUT

–VSY

ISY–

Figure 68. Comparator A

AD8546

1/2

100kΩ

ISY+

+VSY

VOUT

–VSY

ISY–

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,

ISY+ remains at 36 μA per dual amplifier, but ISY− increases to

140 μA in magnitude per dual amplifier.

100

120

140

160

02468

VSY (V)

ISY–

ISY+

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, VDD, whereas

signal ground is in the receiver. The loop current is measured

at the load resistor, RL, at the receiver side.

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