DAC8043

Rev. E | Page 11 of 16

To further ensure accuracy across the full temperature range,

permanently on MOS switches were included in series with

the feedback resistor and the terminating resistor of the R-2R

ladder. The simplified DAC circuit, Figure 13, shows the location

of the series switches. These series switches are equivalently

scaled to two times Switch S1 (MSB) and to Switch S12 (LSB),

respectively, to maintain constant relative voltage drops with

varying temperature. During any testing of the resistor ladder

to turn on these series switches.

20k

Ω

20k

Ω

20k

Ω

GND

S12

S3

20k

Ω

S2

20k

Ω

S1

10k

Ω

10k

Ω

*THESE SWITCHES PERMANENTLY ON.

10k

Ω

10k

Ω

BIT 1 (MSB)

BIT 2

DIGITAL INPUTS

(SWITCHES SHOWN FOR DIGITAL INPUTS (HIGH))

BIT 3

BIT 12 (LSB)

*

*

Figure 13. Simplified DAC Circuit

EQUIVALENT CIRCUIT ANALYSIS

Figure 14 shows an equivalent analog circuit for the DAC8043.

of surface and junction leakages and doubles approximately

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

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

nominal R-2R resistor ladder resistance.

R

GND

R

R

R

Figure 14. Equivalent Analog Circuit

DYNAMIC PERFORMANCE

Output Impedance

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

output capacitance, varies with the digital input code. This

10 kΩ (the feedback resistor alone when all digital inputs are low)

and 7.5 kΩ (the feedback resistor in parallel with approximately

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

logic is high). Static accuracy and dynamic performance will be

affected by these variations. This variation is best illustrated by

using the circuit of Figure 15 and the following equation:













+

=

O

FB

OS

ERROR

R

R

V

V

1

where:

= 10 kΩ for more than four bits of Logic 1.

= 30 kΩ for any single bit of Logic 1.

Therefore, the offset gain varies as follows:

At Code 0011 1111 1111,

OS

OS

1

ERROR

V

V

V

2

kΩ

10

kΩ

10

1

=

















+

=

At Code 0100 0000 0000,

OS

OS

2

ERROR

V

V

V

3

/

4

kΩ

30

kΩ

10

1

=

















+

=

either by using the amplifier’s nulling pins or an external nulling

and OP42.

2R

2R

2R

ETC

R

R

R

OP77

Figure 15. Simplified Circuit

The gain and phase stability of the output amplifier, board

layout, and power supply decoupling all affect the dynamic

performance. The use of a small compensation capacitor may

be required when high speed operational amplifiers are used. It

may be connected across the feedback resistor of the amplifier

to provide the necessary phase compensation to critically damp

resistor form a pole that must be outside the amplifier’s unity

gain crossover frequency.

The considerations when using high speed amplifiers are:

1. Phase compensation (see Figure 16 and Figure 17).

2. Power supply decoupling at the device socket and the use

of proper grounding techniques.