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DAC8043
(Rev.:Rev_C)

12-Bit Serial Input Multiplying CMOS D/A Converter

Analog Devices 

DAC8043

To further insure accuracy across the full temperature range,

permanently “ON” MOS switches were included in series with

the feedback resistor and the R-2R ladder’s terminating resistor.

The “Simplified DAC Circuit,” Figure 2, shows the location of

the series switches. These series switches are equivalently scaled

to two times switch 1 (MSB) and to switch 12 (LSB) respec-

tively to maintain constant relative voltage drops with varying

temperature. During any testing of the resistor ladder or

RFEEDBACK (such as incoming inspection), VDD must be present

to turn “ON” these series switches.

DYNAMIC PERFORMANCE

OUTPUT IMPEDANCE

The DAC8043’s output resistance, as in the case of the output

capacitance, varies with the digital input code. This resistance,

looking back into the IOUT terminal, may be between 10 kΩ (the

feedback resistor alone when all digital inputs are LOW) and

7.5 kΩ (the feedback resistor in parallel with approximate 30 kΩ

of the R-2R ladder network resistance when any single bit logic

is HIGH). Static accuracy and dynamic performance will be af-

fected by these variations.

This variation is best illustrated by using the circuit of Figure 4

and the equation:

VERROR

=

VOS

1

+

RFB

RO





where RO is a function of the digital code, and:

RO = 10 kΩ for more than four bits of logic 1.

RO = 30 kΩ for any single bit of logic 1.

Therefore, the offset gain varies as follows:

at code 0011 1111 1111,

VERROR1

=

VOS

1+1100

kΩ

kΩ

=

2

VOS

at code 0100 0000 0000,

Figure 2. Simplified DAC Circuit

EQUIVALENT CIRCUIT ANALYSIS

Figure 3 shows an equivalent analog circuit for the DAC8043.

The (D × VREF)/R current source is code dependent and is the

current generated by the DAC. The current source ILKG consists

of surface and junction leakages and doubles approximately ev-

ery 10°C. COUT is the output capacitance; it is the result of the

N-channel MOS switches and varies from 80 pF to 110 pF

depending on the digital input code. RO is the equivalent output

resistance that also varies with digital input code. R is the nomi-

nal R-2R resistor ladder resistance.

VERROR2

=

VOS

1 + 1300

kΩ

kΩ

=

4/3

VOS

The error difference is 2/3 VOS.

Since one LSB has a weight (for VREF = +10 V) of 2.4 mV for

the DAC8043, it is clearly important that VOS be minimized,

either using the amplifier’s nulling pins, an external nulling net-

work, or by selection of an amplifier with inherently low VOS.

Amplifiers with sufficiently low VOS include ADI’s OP77, OP07,

OP27, and OP42.

Figure 3. Equivalent Analog Circuit

Figure 4. Simplified Circuit

REV. C

–7–

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