Datasheet

AD5620/AD5640/AD5660 Data Sheet
Rev. G | Page 22 of 28
04539-052
SERIAL
LOAD
AD5660
V
LOOP
12V TO 36V
4–20mA
AD8627
R1
4.7kΩ
R2
18.5kΩ
P1
20mA
ADJUST
P2
4mA
ADJUST
R6
3.3kΩ
R3
1.5kΩ
D1
Q1
2N3904
R7
100Ω
RL
ADR02
Figure 52. Programmable 4 mA to 20 mA Process Controller
USING THE AD5660 AS AN ISOLATED,
PROGRAMMABLE, 4 mA TO 20 mA PROCESS
CONTROLLER
In many process-control system applications, 2-wire current
transmitters are used to transmit analog signals through noisy
environments. These current transmitters use a zero-scale signal
current of 4 mA to power the signal conditioning circuitry of
the transmitter. The full-scale output signal in these transmitters
is 20 mA. The converse approach to process control can also be
used, in which a low-power, programmable current source is
used to control remotely located sensors or devices in the loop.
A circuit that performs this function is shown in Figure 52.
Using the AD5660 as the controller, the circuit provides a
programmable output current of 4 to 20 mA, proportional to
the digital code of the DAC. Biasing for the controller is provided
by the ADR02 and requires no external trim for two reasons: first,
the ADR02’s tight initial output voltage tolerance, and second,
the low supply current consumption of both the AD8627 and
the AD5660. The entire circuit, including optocouplers, consumes
less than 3 mA from the total budget of 4 mA. The AD8627
regulates the output current to satisfy the current summation
at the noninverting node of the AD8627.
I
OUT
= 1/R7 (V
DAC
× R3/R1 + V
REF
× R3/R2)
For the values shown in Figure 52,
I
OUT
= 0.2435 µA × D + 4 mA
where D = 0 ≤ D ≤ 65,535, giving a full-scale output current of
20 mA when the AD5660’s digital code equals 0xFFFF. Offset
trim at 4 mA is provided by P2, and P1 provides the circuit gain
trim at 20 mA. These two trims do not interact because
the noninverting input of the AD8627 is at virtual ground. The
Schottky diode, D1, is required in this circuit to prevent loop
supply power-on transients from pulling the noninverting input
of the AD8627 more than 300 mV below its inverting input.
Without this diode, such transients could cause phase reversal
of the AD8627 and possible latch-up of the controller. The loop
supply voltage compliance of the circuit is limited by the maximum
applied input voltage to the ADR02 and is from 12 V to 40 V.
USING THE AD5620/AD5640/AD5660 WITH A
GALVANICALLY ISOLATED INTERFACE
For process-control applications in industrial environments, it
is often necessary to use a galvanically isolated interface to
protect and isolate the controlling circuitry from hazardous
common-mode voltages that might occur in the area where
the DAC is functioning. The iCoupler® provides isolation in
excess of 2.5 kV. The AD5620/AD5640/AD5660 use a 3-wire
serial logic interface; therefore, the ADuM1300 3-channel
digital isolator provides the required isolation (see Figure 53).
The power supply to the part also must be isolated, which is
done by using a transformer. On the DAC side of the trans-
former, a 5 V regulator provides the 5 V supply required for the
AD5620/AD5640/AD5660.
0.1µF
5V
REGULATOR
GND
04539-053
DIN
SYNC
SCLK
POWER
10µF
SDI
SCLK
DATA
AD56x0
V
OUT
V
OB
V
OA
V
OC
V
DD
V
1C
V
1B
V
1A
ADuM1300
Figure 53. AD5620/AD5640/AD5660 with a Galvanically Isolated Interface