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AD7545AUE

CMOS 12-Bit Buffered Multiplying DAC

Analog Devices 

AD7545A

The circuits of Figures 4, 5 and 6 can all be converted to single

supply operation by biasing AGND to some voltage between

VDD and DGND. Figure 10 shows the 2s Complement Bipolar

circuit of Figure 5 modified to give a range from +2 V to +8 V

about a “pseudo-analog ground” of 5 V. This voltage range

would allow operation from a single VDD of +10 V to +15 V.

The AD584 pin-programmable reference fixes AGND at +5 V.

VIN is set at +2 V by means of the series resistors R1 and R2.

There is no need to buffer the VREF input to the AD7545A with

an amplifier because the input impedance of the D/A converter

is constant. Note, however, that since the temperature coefficient

of the D/A reference input resistance is typically –300 ppm/°C,

applications which experience wide temperature variations may

require a buffer amplifier to generate the +2.0 V at the AD7545A

VREF pin. Other output voltage ranges can be obtained by changing

R4 to shift the zero point and (R1 + R2) to change the slope, or

gain of the D/A transfer function. VDD must be kept at least 5 V

above OUT1 to ensure that linearity is preserved.

Figure 12 shows an alternative approach for use with 8-bit pro-

cessors which have a full 16-bit wide address bus such as 6800,

8080, Z80. This technique uses the 12 lower address lines of the

processor address bus to supply data to the DAC, thus each

AD7545A connected in this way uses 4k bytes of address loca-

tions. Data is written to the DAC using a single memory write

instruction. The address field of the instruction is organized so

that the lower 12 bits contain the data for the DAC and the

upper 4 bits contain the address of the 4k block at which the

DAC resides.

Figure 10. Single Supply "Bipolar" 2s Complement D/A

Converter

MICROPROCESSOR INTERFACING OF THE AD7545A

The AD7545A can interface directly to both 8- and 16-bit

microprocessors via its 12-bit wide data latch using standard CS

and WR control signals.

A typical interface circuit for an 8-bit processor is shown in

Figure 11. This arrangement uses two memory addresses, one

for the lower 8 bits of data to the DAC and one for the upper 4

bits of data into the DAC via the latch.

Figure 12. Connecting the AD7545A to 8-Bit Processors

via the Address Bus

SUPPLEMENTAL APPLICATION MATERIAL

For further information on CMOS multiplying D/A converters

the reader is referred to the following texts:

Reference 1

CMOS DAC Application Guide available from Analog Devices,

Publication Number G872a-15-4/86.

Reference 2

Gain Error and Gain Temperature Coefficient of CMOS

Multiplying DACs – Application Note, Publication Number

E630c–5–3/86.

Reference 3

Analog-Digital Conversion Handbook (Third Edition) available

from Prentice-Hall.

Figure 11. 8-Bit Processor to AD7545 Interface

REV. C

–7–

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