AD420

Rev. H | Page 9 of 16

THEORY OF OPERATION

The AD420 uses a sigma-delta (Σ-Δ) architecture to carry out

the digital-to-analog conversion. This architecture is particularly

well suited for the relatively low bandwidth requirements of the

industrial control environment because of its inherent

monotonicity at high resolution.

In the AD420 a second order modulator is used to keep com-

plexity and die size to a minimum. The single bit stream from

the modulator controls a switched current source that is then

filtered by two, continuous time resistor-capacitor sections.

The capacitors are the only external components that have to be

added for standard current-out operation. The filtered current

is amplified and mirrored to the supply rail so that the application

simply sees a 4 mA–20 mA, 0 mA–20 mA, or 0 mA–24 mA

current source output with respect to ground. The AD420

is manufactured on a BiCMOS process that is well suited to

implementing low voltage digital logic with high performance

and high voltage analog circuitry.

The AD420 can also provide a voltage output instead of a current

loop output if desired. The addition of a single external amplifier

allows the user to obtain 0 V–5 V, 0 V–10 V, ±5 V, or ±10 V.

The AD420 has a loop fault detection circuit that warns if the

to an open-loop circuit or insufficient power supply voltage. The

FAULT DETECT is an active low open drain signal so that one

can connect several AD420s together to one pull-up resistor for

global error detection. The pull-up resistor can be tied to the

internal amplifier as shown in the functional block diagram.

The internal circuitry that develops the fault output avoids

using a comparator with window limits since this would require

an actual output error before the FAULT DETECT output

becomes active. Instead, the signal is generated when the

internal amplifier in the output stage of the AD420 has less than

approximately one volt remaining of drive capability (when

the gate of the output PMOS transistor nearly reaches ground).

Thus the FAULT DETECT output activates slightly before the

compliance limit is reached. Since the comparison is made

within the feedback loop of the output amplifier, the output

accuracy is maintained by its open-loop gain, and no output

error occurs before the fault detect output becomes active.

The 3-wire digital interface, comprising DATA IN, CLOCK,

and LATCH, interfaces to all commonly used serial micropro-

cessors without the addition of any external glue logic. Data is

loaded into an input register under control of CLOCK and is

loaded to the DAC when LATCH is strobed. If a user wants to

minimize the number of galvanic isolators in an intrinsically

safe application, the AD420 can be configured to run in

asynchronous mode. This mode is selected by connecting the

must then be combined with a start and stop bit to frame the

information and trigger the internal LATCH signal.

FAULT

DETECT

BOOST

40Ω

1.25kΩ

4kΩ

OFFSET

TRIM

CAP 1 CAP 2

GND

REF OUT

REF IN

DATA OUT

CLEAR

LATCH

CLOCK

DATA IN

RANGE

SELECT 1

RANGE

SELECT 2

AD420

REFERENCE

DATA I/P

REGISTER

SWITCHED

CURRENT

SOURCES

AND

FILTERING

CLOCK

16-BIT

DAC

2

19

23

18

17

3

21

11

20

16

14

15

10

6

7

8

9

5

4

Figure 5. Functional Block Diagram