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THEORY OF OPERATION

CIRCUIT OPERATION

Digital-to-Analog Converters

R

R

R

2R

2R

2R

R

5 kΩ

S2

S1

DGND

Digital interface onnections are omitted for clarity.

Switches S1 and S2 are closed; V

must be powered.

DD

c

V

OUT

D

16384

(1)

DAC8802

SBAS351A – AUGUST 2005 – REVISED DECEMBER 2005

The DAC8802 contains two 14-bit, current-output, digital-to-analog converters (DACs). Each DAC has its own

independent multiplying reference input. The DAC8802 uses a 3-wire, SPI-compatible serial data interface, with a

configurable asynchronous RS pin for half-scale (MSB = 1) or zero-scale (MSB = 0) preset. In addition, an LDAC

strobe enables two-channel simultaneous updates for hardware-synchronized output voltage changes.

The DAC8802 contains two current-steering R-2R ladder DACs. Figure 30 shows a typical equivalent DAC. Each

14-bit accuracy.

Figure 30. Typical Equivalent DAC Channel

used by the logic to drive the DAC switches on and off. Note that a matching switch is used in series with the

internal 5 k

Ω feedback resistor. If users are attempting to measure the value of R

according to Equation 1:

The DAC is also designed to accommodate ac reference input signals. The DAC8802 accommodates input

reference voltages in the range of -15 V to +15 V. The reference voltage inputs exhibit a constant nominal input

resistance of 5 k

Ω, ±20%. On the other hand, DAC outputs I

various output resistances and capacitances.

The choice of external amplifier should take into account the variation in impedance generated by the DAC8802

on the amplifiers' inverting input node. The feedback resistance, in parallel with the DAC ladder resistance,

dominates output voltage noise. For multiplying mode applications, an external feedback compensation capacitor

voltages.

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