AD7303
–12–
REV. 0
AD7303 to Microwire Interface
Figure 29 shows an interface between the AD7303 and any
microwire compatible device. Serial data is shifted out on the
falling edge of the serial clock and is clocked into the AD7303
on the rising edge of the SK.
SCLK
MICROWIRE*
SK
*ADDITIONAL PINS OMITTED FOR CLARITY
SYNC
DIN
SO
AD7303*
CS
Figure 29. AD7303 to Microwire Interface
APPLICATIONS
Typical Application Circuit
Figure 30 shows a typical setup for the AD7303 when using an
external reference. The reference range for the AD7303 is from
1 V to VDD/2 V. Higher values of reference can be incorporated
but will saturate the output at both the top and bottom end of
the transfer function. From input to output on the AD7303
there is a gain of two. Suitable references for 5 V operation are
the AD780 and REF192. For 3 V operation, a suitable external
reference would be the AD589, a 1.23 V bandgap reference.
AD7303
VOUTA
VOUTB
10F
0.1F
VDD = +3V TO +5V
VDD
GND
AD780/ REF192
WITH VDD = +5V
OR
AD589 WITH VDD = +3V
REF
SCLK
DIN
SYNC
GND
VOUT
VIN
0.1F
SERIAL
INTERFACE
EXT
REF
Figure 30. AD7303 Using External Reference
The AD7303 can also be used with its own internally derived
VDD/2 reference. Reference selection is through the INT/EXT
bit of the 16-bit input word. The internal reference, when
selected, is also provided as an output at the REF pin and can
be decoupled at this point with a 0.1
F capacitor for noise
reduction purposes. AC references can also be applied as exter-
nal references to the AD7303. The AD7303 has limited multi-
plying capability, and a multiplying bandwidth of up to 10 kHz
is achievable.
Bipolar Operation Using the AD7303
The AD7303 has been designed for single supply operation, but
bipolar operation is achievable using the circuit shown in Figure
31. The circuit shown has been configured to achieve an output
voltage range of –5 V < VO < +5 V. Rail-to-rail operation at the
amplifier output is achievable using an AD820 or OP295 as the
output amplifier.
R1
10k
R4
20k
R3
10k
+5V
–5V
±5V
AD7303
VOUTA
10F
0.1F
VDD = +5V
VDD
GND
AD780/ REF192
WITH VDD = +5V
OR
AD589 WITH VDD = +3V
REF
SCLK
DIN
SYNC
GND
VOUT
VIN
0.1F
SERIAL
INTERFACE
EXT
REF
R2
20k
Figure 31. Bipolar Operation Using the AD7303
The output voltage for any input code can be calculated as
follows:
VO = [(1+R4/R3)*(R2/(R1+R2)*(2*VREF*D/256)] – R4*VREF/R3
where
D is the decimal equivalent of the code loaded to the DAC
and
VREF is the reference voltage input.
With VREF = 2.5 V, R1 = R3 = 10 k
and R2 = R4 = 20K and
VDD = 5 V.
VOUT = (10 × D/256) – 5
Opto-Isolated Interface for Process Control Applications
The AD7303 has a versatile 3-wire serial interface making it
ideal for generating accurate voltages in process control and
industrial applications. Due to noise, safety requirements or dis-
tance, it may be necessary to isolate the AD7303 from the con-
troller. This can easily be achieved by using opto-isolators,
which will provide isolation in excess of 3 kV. The serial loading
structure of the AD7303 makes it ideally suited for use in opto-
isolated applications. Figure 32 shows an opto-isolated interface
to the AD7303 where DIN, SCLK and SYNC are driven from
opto-couplers. In this application the reference for the AD7303
is the internal VDD/2 reference. It is being decoupled at the REF
pin with a 0.1
F ceramic capacitor for noise reduction purposes.