AD9740 datasheet(10/20 Pages) AD

10 / 20 page

REV. 0

AD9740

–10–

REFERENCE CONTROL AMPLIFIER

The AD9740 contains a control amplifier that is used to regu-

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

is configured as a V-I converter as shown in Figure 4, 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

(refer to 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 AD9740 provide complementary current

outputs, IOUTA and IOUTB. IOUTA will provide a near full-

scale current output, IOUTFS, when all bits are high (i.e., 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

OUTFS

=×

)

1024

(1)

IOUTB

DAC CODE

OUTFS

=×

(–

) /

1023

1024

(2)

where DAC CODE = 0 to 1023 (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:

OUTFS

REF

=×

(3)

where

REF

REFIO

SET

(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,

RLOAD may represent the equivalent load resistance seen by

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

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

at the IOUTA and IOUTB nodes is simply:

IOUTA

OUTA

LOAD

=×

(5)

IOUTB

OUTB

LOAD

=×

(6)

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

the specified output compliance range to maintain specified

distortion and linearity performance.

IOUTA

IOUTB

DIFF

LOAD

=×

(–

)

(7)

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

be expressed as:

DAC CODE

DIFF

LOAD

SET

REFIO

=×

{}

()×

(–

) /

1023

1024

(8)

These last two equations highlight some of the advantages of

operating the AD9740 differentially. First, the differential

operation will help cancel common-mode error sources associ-

ated 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, the gain drift temperature performance for a single-ended

(VOUTA and VOUTB) or differential output (VDIFF) 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, may be configured for single-ended or differential opera-

tion. IOUTA and IOUTB can be converted into complementary

single-ended voltage outputs, VOUTA and VOUTB, via a load resis-

tor, 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 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

transformer or differential amplifier. These common-mode error

sources include even-order distortion products and noise. The

enhancement in distortion performance becomes more signifi-

cant 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 trans-

former also provides the ability to deliver twice the reconstructed

signal power to the load (i.e., assuming no source termination).

Since the output currents of IOUTA and IOUTB are comple-

mentary, they become additive when processed differentially. A

properly selected transformer will allow the AD9740 to provide

the required power and voltage levels to different loads.

The output impedance of IOUTA and IOUTB is determined

by the equivalent parallel combination of the PMOS switches

associated with the current sources and is typically 100 k

Ω in

parallel with 5 pF. It is also slightly dependent on the output

voltage (i.e., VOUTA and VOUTB) due to the nature of a PMOS

device. As a result, maintaining IOUTA and/or IOUTB at a

virtual ground via an I-V op amp configuration will result in

the optimum dc linearity. Note the INL/DNL specifications

for the AD9740 are measured with IOUTA maintained at a

virtual ground via an op amp.

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