DAC16

REV. B

–8–

reference amplifier. Completely removing the compensation

network would introduce large linearity errors, reference amplifier

instability, wideband reference amplifier noise, and poor settling

time.

Because the DAC exhibits an internal current scaling factor of

eight times (8

×), the reference amplifier requires only 500 µA

input current from the user-supplied precision reference for a

4 mA full-scale output current. In applications that do not re-

quire such high output currents, good accuracy can be achieved

with input reference currents in the range of 350

µA ≤ I

REF

≤ 625 µA. The best signal-to-noise ratios, of course, will be

achieved with a 625

µA reference current which yields a maxi-

mum 5 mA output current. Figure 22 illustrates how to form

the reference input current with a REF02 and a 10 k

Ω precision

resistor.

REF GND

10k

+15V

0.1 F

DAC16

REF02

Figure 22. Generating the DAC16’s Reference Input Current

Reducing Voltage Reference Noise

In data converters of 16-bit and greater resolution, noise is of

critical importance. Surprisingly, the integrated voltage refer-

ence circuit used may contribute the dominant share of a

system’s noise floor, thereby degrading system dynamic range

and signal-to-noise ratio. To maximize system dynamic range

and SNR, all external noise contributions should be effectively

much less than 1/2 LSB. For example, in a 5 V DAC16 applica-

tion, one LSB is equivalent to 76

µV. This means that the total

wideband noise contribution due to a voltage reference and all

other sources should be less than 38

µV rms. These noise levels

are not easy targets to hit with standard off-the-shelf reference

devices. For example, commercially available references might

exhibit 5

µV rms noise from 0.1 Hz to 10 Hz: but, over a 100 kHz

bandwidth, its 300

µV rms of noise can easily swamp out a

16-bit system. Such noisy behavior can degrade a DAC’s effec-

tive resolution by increasing its differential nonlinearity which,

in turn, can lead to nonmonotonic behavior or analog errors.

The easiest way to reduce noise in the reference circuit is to

band-limit its noise before feeding it to the converter. In the

case of the DAC16, the reference is not a voltage, but a current.

Illustrated in Figure 23 is a simple way of hand-limiting

REF GND

R1

5k

+15V

0.1 F

DAC16

REF02

C1

22 F

R2

5k

AGND

Figure 23. Filtering a Reference’s Wideband Noise

and bypassing the common node with a capacitor. To minimize

thermally induced errors, R1 and R2 must be electrically and

thermally well-matched. Thin-film resistor networks work well

here. In this circuit, the parallel combination of R1 and R2

forms a 3 Hz low-pass filter with C1. The only noise source that

remains is the thermal noise of R2 which can be a significantly

lower noise generator than the voltage reference.

Input Coding

The unipolar digital input coding of the DAC16 employs nega-

tive logic to control the output current; that is, an all zero input

current output. An expression for the DAC16’s transfer equa-

tion can be expressed by:

I

65,535 –

Digital Code

65,536

















Table II provides the relationship between the digital input

codes and the output current of the DAC16.

Table II. Unipolar Code Table

Digital Input

DAC16 Output

Word (Hex)

Comment

0000

8

REF

Full Scale

7FFE

8

REF

Midscale + 1 LSB

7FFF

8

REF

Midscale

8000

8

REF

Midscale – 1 LSB

FFFF

0

Zero Scale

Since the DAC16 exhibits a small output voltage compliance on

the order of a few millivolts, a high accuracy operational ampli-

fier must be used to convert the DAC’s output current to a volt-

age. Refer to the section on selecting operation amplifiers for the

DAC16. The circuit shown in Figure 24 illustrates a unipolar

output configuration. In symbolic form, the transfer equation

for this circuit can be expressed by:

V

65,535 –

Digital Code

65,536









In this example, the reference input current was set to 500

µA

which produces a full-scale output current of 4 mA – 1 LSB.

The DAC’s output current was scaled by R3, a 1.25 k

Ω resistor,

to produce a 5 V full-scale output voltage. Bear in mind that to

ensure the highest possible accuracy, matched thin-film resistor

networks are almost a necessity, not an option. The resistors

used in the circuit must have close tolerance and tight thermal

tracking. Table III illustrates the relationship between the input

digital code and the circuit’s output voltage for the component

values shown.

Table III. Unipolar Output Voltage vs. Digital Input Code

Digital Input Word

Decimal Number in

Analog Output

(Hex)

in DAC Decoder

Voltage (V)

0000

65,535

4.999924

7FFE

32,769

2.500076

7FFF

32,768

2.500000

8000

32,767

2.499924

FFFF

0

0