20 ANALOG APPLICATIONS

The analog output signal for these DACs is derived from a

conventional R-2R current switching ladder network A de-

tailed description of this network can be found on the

vides a current directly proportional to the product of the

applied reference voltage and the digital input word A sec-

plement of the digital input Specifically

R

c

D

4096

R

c

4096

where D is the decimal equivalent of the applied 12-bit bina-

the R-2R ladder R is nominally 15 kX

21 Obtaining a Unipolar Output Voltage

To maintain linearity of output current with changes in the

applied digital code it is important that the voltages at both

of the current output pins be as near ground potential (0

error In most applications this output current is converted to

a voltage by using an op amp as shown in

Figure 9

The inverting input of the op amp is a virtual ground created

by the feedback from its output through the internal 15 kX

to the output of the amplifier Two-quadrant operation can

of the amplifier The output voltage in either case is always

erence voltage

The reference can be either a stable DC voltage source or

The DAC can be thought of as a digitally controlled attenua-

tor the output voltage is always less than the applied refer-

nominal impedance of 15 kX to ground to external circuitry

voltage since this resistor matches (and tracks with temper-

ature) the value of the resistors used to generate the output

The selected op amp should have as low a value of input

bias current as possible The product of the bias current

times the feedback resistance creates an output voltage er-

ror which can be significant in low reference voltage appli-

with these DACs because of their very low input current

TLH5690 – 15

e

b

4096

FIGURE 9 Unipolar Output Configuration

12