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THEORY OF OPERATION

R

R

R

R

2R

2R

2R

2R

2R

DB0

(LSB)

DB9

DB10

DB11

(MSB)

I

1

OUT

R

FB

V

REF

I

2

OUT

V

OUT

+ * VREF

CODE

4096

(1)

OPA277

DAC7821

15V

V+

V

-

-

15V

GND

I

1

OUT

R

FB

V

REF

V

DD

V

DD

V

OUT

I

2

OUT

DAC7821

SBAS365B – OCTOBER 2005 – REVISED JULY 2007

The DAC7821 is a single channel, current output, 12-bit digital-to-analog converter (DAC). The architecture,

illustrated in Figure 25, is an R-2R ladder configuration with the three MSBs segmented. Each 2R leg of the

ladder is either switched to IOUT1 or the IOUT2 terminal. The IOUT1 terminal of the DAC is held at a virtual GND

potential by the use of an external I/V converter op amp. The R-2R ladder is connected to an external reference

input VREF that determines the DAC full-scale current. The R-2R ladder presents a code-independent load

impedance to the external reference of 10k

Ω ±20%. The external reference voltage can vary over a range of

–15V to +15V, thus providing bipolar IOUT current operation. By using an external I/V converter and the

DAC7821 RFB resistor, output voltage ranges of –VREF to VREF can be generated.

Figure 25. Equivalent R-2R DAC Circuit

When using an external I/V converter and the DAC7821 RFB resistor, the DAC output voltage is given by

Equation 1:

Each DAC code determines the 2R leg switch position to either GND or IOUT. Because the DAC output

impedance as seen looking into the IOUT1 terminal changes versus code, the external I/V converter noise gain

will also change. Because of this, the external I/V converter op amp must have a sufficiently low offset voltage

such that the amplifier offset is not modulated by the DAC IOUT1 terminal impedance change. External op amps

with large offset voltages can produce INL errors in the transfer function of the DAC7821 as a result of offset

modulation versus DAC code.

For best linearity performance of the DAC7821, an op amp with a low input offset voltage (OPA277) is

recommended (see Figure 26). This circuit allows VREF swinging from –10V to +10V.

Figure 26. Voltage Output Configuration

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