AD9774

–13–

REV. B

50pF

+1.2V REF

AVDD

REFLO

CURRENT

SOURCE

ARRAY

REFIO

FS ADJ

AD9774

EXTERNAL

REF

AVDD

REFERENCE

CONTROL

AMPLIFIER

0.1 F

REFCOMP

Figure 28. External Reference Configuration

REFERENCE CONTROL AMPLIFIER

The AD9774 also contains an internal control amplifier that is

The control amplifier is configured as a V-I converter, as shown

Equation 3.

The control amplifier allows a wide (10:1) adjustment span of

62.5

µA and 625 µA. The wide adjustment span of I

OUTFS

provides several application benefits. The first benefit relates

directly to the power dissipation of the AD9774, which is pro-

second benefit relates to the 20 dB adjustment, which is useful

for system gain control purposes.

which the internal reference is disabled, and the common-mode

voltage of REFIO is varied over its compliance range of 1.25 V

to 0.10 V. REFIO can be driven by a single-supply amplifier or

input impedance of REFIO is approximately 1 M

Ω, a simple,

low cost R-2R ladder DAC configured in the voltage mode

topology may be used to control the gain. This circuit is shown

in Figure 30 using the AD7524 and an external 1.2 V reference,

the AD1580.

The second method may be used in a dual-supply system in

fier. An example of this method is shown in Figure 29 in which

the internal reference is used to set the common-mode voltage

referenced to ACOM and should not exceed 1.2 V. The value of

and 625

µA, respectively. The associated equations in Figure 29

50pF

+1.2V REF

AVDD

REFLO

CURRENT

SOURCE

ARRAY

REFIO

FSADJ

AD9774

1 F

625A

0.1 F

REFCOMP

Figure 29. Dual Supply Gain Control Circuit

ANALOG OUTPUTS

The AD9774 produces two complementary current outputs,

IOUTA and IOUTB, which may be configured for single-end or

differential operation. IOUTA and IOUTB can be converted

Transfer Function section by Equations 5 through 8. The

can also be converted to a single-ended voltage via a transformer

or differential amplifier configuration.

Figure 31 shows the equivalent analog output circuit of the

AD9774 consisting of a parallel combination of PMOS differen-

tial current switches associated with each segmented current

source. The output impedance of IOUTA and IOUTB is deter-

mined by the equivalent parallel combination of the PMOS

switches and is typically 100 k

Ω in parallel with 5 pF. Due to

the nature of a PMOS device, the output impedance is also

and, to a lesser extent, the analog supply voltage, AVDD, and

signal dependency can be a source of dc nonlinearity and ac linear-

ity (i.e., distortion), its effects can be limited if certain precau-

tions are noted.

1.2V

50pF

+1.2V REF

AVDD

REFLO

CURRENT

SOURCE

ARRAY

REFIO

FSADJ

AD9774

AVDD

OUT1

OUT2

AGND

DB7–DB0

AD7524

AD1580

0.1V TO 1.2V

0.1 F

REFCOMP

Figure 30. Single Supply Gain Control Circuit