AD9742ARURL7 datasheet(13/32 Pages) AD

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Data Sheet

AD9742

Rev. C | Page 13 of 32

REFERENCE CONTROL AMPLIFIER

The AD9742 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 AD9742, which is proportional to IOUTFS (see

the Power Dissipation section). The second 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 500 kHz and can be used for low frequency small

signal multiplying applications.

DAC TRANSFER FUNCTION

Both DACs in the AD9742 provide complementary current

outputs, IOUTA and IOUTB. IOUTA provides a near full-scale

current output, IOUTFS, when all bits are high (i.e., DAC CODE =

4095), 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:

(

)

OUTFS

CODE

DAC

IOUTA

4096

(1)

(

)

OUTFS

CODE

DAC

IOUTB

−

/4096

4095

(2)

where DAC CODE = 0 to 4095 (i.e., 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:

REF

OUTFS

= 32

(3)

where

SET

REFIO

REF

(4)

The two current outputs will typically drive a resistive load

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

and IOUTB should be directly connected to matching resistive

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

RLOAD may 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

LOAD

OUTA

IOUTA

(5)

LOAD

OUTB

IOUTB

(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.

(

)

LOAD

DIFF

IOUTB

IOUTA

−

(7)

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

expressed as:

(

)

{

}

(

)

REFIO

SET

LOAD

DIFF

CODE

DAC

−

4096

4095

(8)

Equations 7 and 8 highlight some of the advantages of operating

the AD9742 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 (i.e.,

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 (VDIFF) of the

AD9742 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 may be configured for single-ended or differential

operation. IOUTA and IOUTB can be converted into comple-

mentary single-ended voltage outputs, VOUTA and VOUTB, via a

load resistor, RLOAD, as described in the DAC Transfer Function

section by Equations 5 through 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 AD9742 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 AD9742 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

transformer or differential amplifier. These common-mode

error sources include even-order distortion products and noise.

The enhancement in distortion performance becomes more

significant as the frequency content of the reconstructed

waveform increases and/or its amplitude decreases. This is due

to the first-order cancellation of various dynamic common-

mode distortion mechanisms, digital feedthrough, and noise.

Performing a differential-to-single-ended conversion via a

transformer also provides the ability to deliver twice the

reconstructed signal power to the load (assuming no source

termination). Since the output currents of IOUTA and IOUTB

are complementary, they become additive when processed

differentially. A properly selected transformer will allow the

AD9742 to provide the required power and voltage levels to

different loads.

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