AD9740-EB datasheet(14/32 Pages) AD

14 / 32 page

AD9740

Rev. B | Page 14 of 32

150pF

1.2V REF

AVDD

REFLO

CURRENT

SOURCE

ARRAY

REFIO

FS ADJ

AD9740

REFERENCE

CONTROL

AMPLIFIER

3.3V

Figure 25. External Reference Configuration

REFERENCE CONTROL AMPLIFIER

The AD9740 contains a control amplifier that is used to regulate

the full-scale output current, IOUTFS. The control amplifier is

configured as a V-I converter, as shown in Figure 24, so 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

to the segmented current sources with the proper scale 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 IOUTFS

provides several benefits. The first relates directly to the power

dissipation of the AD9740, which is proportional to IOUTFS (see

the Power Dissipation section). The second relates to a 20 dB

adjustment, which is useful for system gain control purposes.

The small signal bandwidth of the reference control amplifier is

approximately 500 kHz and can be used for low frequency small

signal multiplying applications.

DAC TRANSFER FUNCTION

The AD9740 provides complementary current outputs, IOUTA

and IOUTB. IOUTA provides a near full-scale current output,

IOUTFS, when all bits are high (that is, DAC CODE = 1023), while

IOUTB, the complementary output, provides no current. The

current output appearing at IOUTA and IOUTB is a function of

both the input code and IOUTFS and can be expressed as:

IOUTA = (DAC CODE/1023) × IOUTFS

(1)

IOUTB = (1023 − DAC CODE)/1024 × IOUTFS

(2)

where DAC CODE = 0 to 1023 (that is, decimal representation).

As mentioned previously, IOUTFS is a function of the reference

current IREF, which is nominally set by a reference voltage,

VREFIO, and external resistor, RSET. It can be expressed as:

IOUTFS = 32 × IREF

(3)

where

IREF = VREFIO/RSET

(4)

The two current outputs typically drive a resistive load directly

or via a transformer. If dc coupling is required, then IOUTA

and IOUTB should be directly connected to matching resistive

loads, RLOAD, that are tied to analog common, ACOM. Note that

RLOAD can represent the equivalent load resistance seen by

IOUTA or IOUTB, as would be the case in a doubly terminated

50 Ω or 75 Ω cable. The single-ended voltage output appearing

at the IOUTA and IOUTB nodes is simply

VOUTA = IOUTA × RLOAD

(5)

VOUTB = IOUTB × RLOAD

(6)

Note that the full-scale value of VOUTA and VOUTB should not

exceed the specified output compliance range to maintain

specified distortion and linearity performance.

VDIFF = (IOUTA − IOUTB) × RLOAD

(7)

Substituting the values of IOUTA, IOUTB, IREF, and VDIFF can be

expressed as:

VDIFF = {(2 × DAC CODE − 1023)/1024}

(32 × RLOAD/RSET) × VREFIO

(8)

Equation 7 and Equation 8 highlight some of the advantages of

operating the AD9740 differentially. First, the differential

operation helps cancel common-mode error sources associated

with IOUTA and IOUTB, such as noise, distortion, and dc

offsets. Second, the differential code-dependent current and

subsequent voltage, VDIFF, is twice the value of the single-ended

voltage output (that is, VOUTA or VOUTB), thus providing twice the

signal power to the load.

Note that the gain drift temperature performance for a single-

ended (VOUTA and VOUTB) or differential output (V

DIFF

) of the

AD9740 can be enhanced by selecting temperature tracking

resistors for RLOAD and RSET due to their ratiometric relationship,

as shown in Equation 8.

ANALOG OUTPUTS

The complementary current outputs in each DAC, IOUTA,

and IOUTB can be configured for single-ended or differential

operation. IOUTA and IOUTB can be converted into

complementary single-ended voltage outputs, VOUTA and VOUTB,

via a load resistor, RLOAD, as described in the DAC Transfer

Function section by Equation 5 through Equation 8. The

differential voltage, VDIFF, existing between VOUTA and VOUTB, can

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

differential amplifier configuration. The ac performance of the

AD9740 is optimum and specified using a differential

transformer-coupled output in which the voltage swing at

IOUTA and IOUTB is limited to ±0.5 V.

The distortion and noise performance of the AD9740 can be

enhanced when it is configured for differential operation. The

common-mode error sources of both IOUTA and IOUTB can

be significantly reduced by the common-mode rejection of a

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