AD5620 datasheet(21/24 Pages) AD

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AD5620/AD5640/AD5660

Rev. A | Page 21 of 24

APPLICATIONS

USING AN REF19x AS A POWER SUPPLY FOR THE

AD5620/AD5640/AD5660

Because the supply current required by the AD5620/AD5640/

AD5660 is extremely low, an alternative option is to use a REF19x

voltage reference (REF195 for 5 V or REF193 for 3 V) to supply

the required voltage to the part—see Figure 49. This is especially

useful if the power supply is quite noisy or if the system supply

voltages are at some value other than 5 V or 3 V, for example, 15 V.

The REF19x outputs a steady supply voltage for the AD5620/

AD5640/AD5660. If the low dropout REF195 is used, the current

it needs to supply to the AD5660 is 500 μA. This is with no load

on the output of the DAC. When the DAC output is loaded, the

REF195 also must supply the current to the load. The total current

required (with a 5 kΩ load on the DAC output) is

500 μA + (5 V/5 kΩ) = 1.5 mA

The load regulation of the REF195 is typically 2 ppm/mA,

which results in an error of 3 ppm (15 μV) for the 1.5 mA

current drawn from it. This corresponds to a 0.197 LSB error

for the AD5660.

AD5660

3-WIRE

SERIAL

INTERFACE

SYNC

SCLK

DIN

15V

VOUT = 0V TO 5V

REF195

Figure 49. REF195 as the Power Supply to the AD5660

BIPOLAR OPERATION USING THE AD5660

The AD5660 is designed for single-supply operation, but a

bipolar output range is also possible using the circuit in

Figure 50. Figure 50 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

⎥⎦

⎤

⎢⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

−

⎟

⎠

⎞

⎜

⎝

⎛

⎟

⎠

⎞

⎜

⎝

⎛

65536

where D represents the input code in decimal (0 to 65,535).

When VDD = 5 V, R1 = R2 = 10 kΩ,

65536

−

⎟

⎠

⎞

⎜

⎝

⎛ ×

This results in an output voltage range of ±5 V, with 0x0000

corresponding to a −5 V output and 0xFFFF corresponding to a

+5 V output.

10k

+5V

–5V

AD820/

OP295

3-WIRE

SERIAL

INTERFACE

+5V

AD5660

VDD

VFB

VOUT

10k

±5V

0.1

μF

Figure 50. Bipolar Operation with the AD5660

USING THE AD5660 AS AN ISOLATED,

PROGRAMMABLE, 4 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.

IOUT = 1/R7 (VDAC × R3/R1 + VREF × R3/R2)

For the values shown in Figure 51,

IOUT = 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

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