AD5620 Analog Devices, AD5620 Datasheet - Page 22

• Manufacturer Part Number: AD5620
• Manufacturer: Analog Devices

Specifications of AD5620

• Resolution (bits): 12bit
• Dac Update Rate: 125kSPS
• Dac Settling Time: 8µs
• Max Pos Supply (v): +5.5V
• Single-supply: Yes
• Dac Type: Voltage Out
• Dac Input Format: Ser,SPI

AD5620/AD5640/AD5660
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 51.
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.
For the values shown in Figure 51,
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.
I
I
OUT
OUT
= 1/R7 (V
= 0.2435 μA × D + 4 mA
DAC
× R3/R1 + V
SERIAL
LOAD
REF
× R3/R2)
AD5660
Figure 51. Programmable 4 mA to 20 mA Process Controller
4.7k
R1
ADJUST
ADJUST
18.5k
20mA
1.5k
P1
4mA
P2
R2
R3
Rev. F | Page 22 of 28
AD8627
ADR02
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 52).
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.
SCLK
DATA
Figure 52. AD5620/AD5640/AD5660 with a Galvanically Isolated Interface
SDI
POWER
3.3k
R6
100
R7
V
V
V
Q1
2N3904
1A
1B
1C
D1
ADuM1300
4–20mA
V
V
V
OA
OB
OC
12V TO 36V
REGULATOR
RL
V
LOOP
5V
SCLK
SYNC
DIN
AD56x0
GND
V
DD
V
10F
OUT
0.1F