Datasheet

AD5662
Rev. A | Page 19 of 24
BIPOLAR OPERATION USING THE AD5662
The AD5662 has been designed for single-supply operation,
but a bipolar output range is also possible using the circuit in
Figure 42. The circuit gives an output voltage range of ±5 V.
Rail-to-rail operation at the amplifier output is achievable using
an AD820 or an OP295 as the output amplifier.
The output voltage for any input code can be calculated as
follows:
⎥
⎦
⎤
⎢
⎣
⎡
⎟
⎠
⎞
⎜
⎝
⎛
×−
⎟
⎠
⎞
⎜
⎝
⎛
+
×
⎟
⎠
⎞
⎜
⎝
⎛
×=
R1
R2
V
R1
R2R1D
VV
DDDD
O
536,65
where D represents the input code in decimal (0 to 65,535).
With V
DD
= 5 V, R1 = R2 = 10 kΩ,
V5
536,65
10
−
⎟
⎠
⎞
⎜
⎝
⎛
×
=
D
V
O
This is an output voltage range of ±5 V, with 0x0000 corre-
sponding to a −5 V output, and 0xFFFF corresponding to a
+5 V output.
R2 = 10kΩ
04777-032
+5V
–5V
AD820/
OP295
THREE-WIRE
SERIAL
INTERFACE
+5V
AD5662
V
REF
V
OUT
V
FB
R1 = 10kΩ
±5V
0.1μF10μF
Figure 42. Bipolar Operation with the AD5662
USING THE AD5662 AS AN ISOLATED,
PROGRAMMABLE, 4-20 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 that can power the transmitter’s signal
conditioning circuitry. The full-scale output signal in these
transmitters is 20 mA. The converse approach to process
control can also be used; a low-power, programmable current
source can be used to control remotely located sensors or
devices in the loop.
A circuit that performs this function is shown in Figure 43.
Using the AD5662 as the controller, the circuit provides a
programmable output current of 4 mA to 20 mA, proportional
to the DAC’s digital code. Biasing for the controller is provided
by the ADR02 and requires no external trim for two reasons:
(1) the ADR02’s tight initial output voltage tolerance and (2)
the low supply current consumption of both the AD8627 and
the AD5662. The entire circuit, including opto-couplers,
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 43,
I
OUT
= 0.2435 μA × D + 4 mA
where D = 0 ≤ D ≤ 65535, giving a full-scale output current of
20 mA when the AD5662’s digital code equals 0xFFFF. Offset
trim at 4 mA is provided by P2, and P1 provides the circuit’s
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 maxi-
mum applied input voltage to the ADR02 and is from 12 V to
40 V.
04777-034
SERIA
L
LOAD
AD5662
V
LOOP
12V TO 36V
4mA TO 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 43. Programmable 4–20 mA Process Controller