Low-Power, Quad, 12-Bit

Voltage-Output DACs with Serial Interface

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13

Digitally Programmable Current Source

The circuit of Figure 12 places an npn transistor

(2N3904 or similar) within the op-amp feedback loop to

implement a digitally programmable, unidirectional cur-

rent source. This circuit drives 4mA to 20mA current

loops, which are commonly used in industrial-control

applications. The output current is calculated with the

following equation:

where NB is the numeric value of the DAC’s binary input

code and R is the sense resistor shown in Figure 12.

Power-Supply Considerations

On power-up, all input and DAC registers are cleared

shifted out of DOUT on the clock’s falling edge).

capacitor in parallel with a 0.1µF capacitor to AGND.

Use short lead lengths and place the bypass capaci-

tors as close as possible to the supply inputs.

Grounding and Layout Considerations

Digital or AC transient signals between AGND and

DGND create noise at the analog outputs. Connect

AGND and DGND together at the DAC, and then con-

nect this point to the highest-quality ground available.

Good PCB ground layout minimizes crosstalk between

DAC outputs, reference inputs, and digital inputs.

Reduce crosstalk by keeping analog lines away from

digital lines. Do not use wire-wrapped boards.

Chip Information

PROCESS: BiCMOS

DAC

+5V

-5V

R1 = R2 = 10kΩ ± 0.1%

MAX5500

MAX5501

REF_

R1

R2

FB_

OUT_

Figure 11. Bipolar Output Circuit

DAC_

MAX5500

MAX5501

REF_

OUT_

R

2N3904

FB_

Figure 12. Digitally Progammable Current Source