AD668

REV. A

–9–

Figure 8. 1.25 V REFIN/

±500 mV Unbuffered Bipolar

Output

5 V REFIN, 2 V BIPOLAR, UNBUFFERED VOLTAGE

OUTPUT

Figure 9 demonstrates how a larger unbuffered voltage output

(Pin 20) to produce an output resistance of roughly 200

Ω.

Figure 9. 5 V REFIN/

±1 V Unbuffered Bipolar Output

It should be noted that this impedance is not trimmed, and may

vary by as much as 20%, but this can be compensated by adjust-

ing the reference voltage. It is also important to note that limita-

tions in the DAC output compliance would prohibit use of a 2 V

unipolar output voltage swing.

1 V REFIN, –10 V UNIPOLAR, BUFFERED VOLTAGE

OUTPUT

Figure 10 shows the implementation of the 1 V full scale for the

reference input by tying REFIN1 and REFIN2 together and

driving them both with the input voltage. This generates a high

input impedance, and some care should be taken to insure that

the driving impedance at this node is finite at all times to avoid

saturating the reference amplifier. This is typically accomplished

by a using a low impedance voltage source to drive the refer-

ence, but if the topology calls for this source to be switched out,

a high impedance (10 k

Ω) termination resistor should be used

on the REFIN node.

Figure 10. 1 V REFIN/–10 V Unipolar Buffered Output

For full-scale output ranges greater than 2 V, some type of ex-

ternal buffer amplifier is needed. The AD840 fills this require-

ment perfectly, settling to within 0.025% from a 10 V full-scale

step in less than 100 ns. As shown in Figure 10, the amplifier

establishes a summing node at ground for the DAC output. The

output voltage is determined by the amplifier’s feedback resistor

(10.24 V for a 1k resistor). Note that since the DAC generates a

positive current to ground, the voltage at the amplifier output

will be negative. A series resistor between the noninverting am-

plifier input and ground minimizes the offset effects of op amp

input bias currents.

The optimal DAC output impedance in buffered output appli-

cations depends on the buffer amplifier being used. The AD840

is stable at a gain of 10, so a lower DAC output impedance

should be grounded. The 100

Ω DAC output impedance pro-

duces a noise gain of 11 with the 1k feedback resistor. If the

gain-of-two stable AD842 is used as a buffer, a 200

Ω DAC out-

put impedance will produce a stable configuration with lower

the DAC output.

As noted earlier, these four examples are part of an array of

possible configurations available. Table II provides a quick

reference chart for the more straightforward applications, but

many other input and output signals are possible with some

modifications.

The next three circuits provide examples of different analog in-

put drives, including a fixed dc reference, a capacitively coupled

ac reference, and a DAC driving the reference channel. Note

that the entire spectrum of input and output range configura-

tions are available regardless of the type of reference drive being

used.

DC REFERENCE: THE AD586 DRIVING THE AD668

Figure 11 illustrates one of the more obvious analog input

sources: a fixed reference. The AD586 produces a temperature

stable 5 V analog output to drive the AD668 in the 5 V input