Datasheet

Test Circuits (Continued)

Application Hints

REFERENCE AMPLIFIER DRIVE AND COMPENSATION

The reference amplifier provides a voltage at pin 14 for

converting the reference voltage to a current, and a

turn-around circuit or current mirror for feeding the ladder.

The reference amplifier input currrent, I

, must always flow

into pin 14, regardless of the set-up method or reference

voltage polarity.

Connections for a positive voltage are shown in

Figure 7

The reference voltage source supplies the full current I

For bipolar reference signals, as in the multiplying mode,

R15 can be tied to a negative voltage corresponding to the

minimum input level. It is possible to eliminate R15 with only

a small sacrifice in accuracy and temperature drift.

The compensation capacitor value must be increased with

increases in R14 to maintain proper phase margin; for R14

values of 1, 2.5 and 5 kΩ, minimum capacitor values are 15,

37 and 75 pF. The capacitor may be tied to either V

ground, but using V

increases negative supply rejection.

A negative reference voltage may be used if R14 is

grounded and the reference voltage is applied to R15 as

shown in

Figure 8

. A high input impedance is the main

DS005687-11

FIGURE 8. Negative V

REF

(Note 8)

DS005687-12

FIGURE 9. Programmable Gain Amplifier or

Digital Attenuator Circuit (Note 8)

DAC0808