AD9764 datasheet(10/22 Pages) AD

10 / 22 page

REV. B

AD9764

–10–

REFERENCE OPERATION

The AD9764 contains an internal 1.20 V bandgap reference

that can be easily disabled and overridden by an external

reference. REFIO serves as either an input or output, depending

on whether the internal or external reference is selected. If

REFLO is tied to ACOM, as shown in Figure 22, the internal

reference is activated, and REFIO provides a 1.20 V output. In

this case, the internal reference must be compensated externally

with a ceramic chip capacitor of 0.1

µF or greater from REFIO

to REFLO. Also, REFIO should be buffered with an external

amplifier having an input bias current less than 100 nA if any

additional loading is required.

50pF

COMP1

+1.2V REF

AVDD

REFLO

CURRENT

SOURCE

ARRAY

0.1 F

+5V

REFIO

FS ADJ

0.1 F

AD9764

ADDITIONAL

LOAD

OPTIONAL

EXTERNAL

REF BUFFER

Figure 22. Internal Reference Configuration

The internal reference can be disabled by connecting REFLO to

AVDD. In this case, an external reference may then be applied

to REFIO as shown in Figure 23. The external reference may

provide either a fixed reference voltage to enhance accuracy and

drift performance or a varying reference voltage for gain control.

Note that the 0.1

µF compensation capacitor is not required

since the internal reference is disabled, and the high input im-

pedance (i.e., 1 M

Ω) of REFIO minimizes any loading of the

external reference.

50pF

COMP1

+1.2V REF

AVDD

REFLO

CURRENT

SOURCE

ARRAY

0.1 F

AVDD

REFIO

FS ADJ

RSET

AD9764

EXTERNAL

REF

IREF =

VREFIO/RSET

AVDD

REFERENCE

CONTROL

AMPLIFIER

VREFIO

Figure 23. External Reference Configuration

REFERENCE CONTROL AMPLIFIER

The AD9764 also contains an internal control amplifier that is

used to regulate the DAC’s full-scale output current, IOUTFS.

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

in Figure 23, such that its current output, IREF, is determined by

the ratio of the VREFIO and an external resistor, RSET, as stated

in Equation 4. IREF is copied over to the segmented current

sources with the proper scaling factor to set IOUTFS as stated in

Equation 3.

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

IOUTFS over a 2 mA to 20 mA range by setting IREF between

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 AD9764, which is pro-

portional to IOUTFS (refer to the Power Dissipation section). The

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

for system gain control purposes.

The small signal bandwidth of the reference control amplifier is

approximately 1.4 MHz and can be reduced by connecting an

external capacitor between COMP1 and AVDD. The output of

the control amplifier, COMP1, is internally compensated via a

50 pF capacitor that limits the control amplifier small-signal

bandwidth and reduces its output impedance. Any additional

external capacitance further limits the bandwidth and acts as a

filter to reduce the noise contribution from the reference ampli-

fier. Figure 24 shows the relationship between the external

capacitor and the small signal –3 dB bandwidth of the refer-

ence amplifier. Since the –3 dB bandwidth corresponds to the

dominant pole, and hence the time constant, the settling time of

the control amplifier to a stepped reference input response can

be approximated.

COMP1 CAPACITOR – nF

1000

0.1

1000

100

Figure 24. External COMP1 Capacitor vs. –3 dB Bandwidth

The optimum distortion performance for any reconstructed

waveform is obtained with a 0.1

µF external capacitor installed.

Thus, if IREF is fixed for an application, a 0.1

µF ceramic chip

capacitor is recommended. Also, since the control amplifier is

optimized for low power operation, multiplying applications

requiring large signal swings should consider using an external

control amplifier to enhance the application’s overall large signal

multiplying bandwidth and/or distortion performance.

There are two methods in which IREF can be varied for a fixed

RSET. The first method is suitable for a single-supply system in

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

DAC, thus allowing IREF to be varied for a fixed RSET. Since the

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 25 using the AD7524 and an external 1.2 V reference,

the AD1580.

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