Datasheet

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AD9709

Rev. B | Page 14 of 32

DAC TRANSFER FUNCTION

Both DACs in the AD9709 provide complementary current out-

puts, I

OUTA

and I

OUTB

. I

OUTA

provides a near full-scale current

output, I

OUTFS

, when all bits are high (that is, DAC CODE = 256)

while I

OUTB

, the complementary output, provides no current.

The current output appearing at I

OUTA

and I

OUTB

is a function of

both the input code and I

OUTFS

and can be expressed as

OUTA

= (DAC CODE/256) × I

OUTFS

(1)

OUTB

= (255 − DAC CODE)/256 × I

OUTFS

(2)

where DAC CODE = 0 to 255 (that is, decimal representation).

OUTFS

is a function of the reference current (I

REF

), which is

nominally set by a reference voltage (V

REFIO

) and an external

resistor (R

SET

). It can be expressed as

OUTFS

= 32 × I

REF

(3)

where

REF

= V

REFIO

SET

(4)

The two current outputs typically drive a resistive load directly

or via a transformer. If dc coupling is required, I

OUTA

and I

OUTB

should be connected directly to matching resistive loads, R

LOAD

that are tied to the analog common, ACOM. Note that R

LOAD

can represent the equivalent load resistance seen by I

OUTA

OUTB

, as would be the case in a doubly terminated 50 Ω or 75 Ω

cable. The single-ended voltage output appearing at the I

OUTA

and I

OUTB

nodes is

OUTA

= I

OUTA

× R

LOAD

(5)

OUTB

= I

OUTB

× R

LOAD

(6)

Note the full-scale value of V

OUTA

and V

OUTB

must not exceed the

specified output compliance range to maintain the specified

distortion and linearity performance.

DIFF

= (I

OUTA

− I

OUTB

) × R

LOAD

(7)

Equation 7 highlights some of the advantages of operating the

AD9709 differentially. First, the differential operation helps cancel

common-mode error sources associated with I

OUTA

and I

OUTB

such as noise, distortion, and dc offsets. Second, the differential

code-dependent current and subsequent voltage, V

DIFF

, is twice

the value of the single-ended voltage output (that is, V

OUTA

OUTB

), thus providing twice the signal power to the load.

Note that the gain drift temperature performance for a single-

ended (V

OUTA

and V

OUTB

) or differential output (V

DIFF

) of the

AD9709 can be enhanced by selecting temperature tracking

resistors for R

LOAD

and R

SET

due to their ratiometric relationship.

ANALOG OUTPUTS

The complementary current outputs, I

OUTA

and I

OUTB

, in each

DAC can be configured for single-ended or differential

operation. I

OUTA

and I

OUTB

can be converted into complementary

single-ended voltage outputs, V

OUTA

and V

OUTB

, via a load

resistor, R

LOAD

, as described in Equation 5 through Equation 7.

The differential voltage, V

DIFF

, existing between V

OUTA

and V

OUTB

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

differential amplifier configuration. The ac performance of the

AD9709 is optimum and specified using a differential

transformer-coupled output in which the voltage swing at I

OUTA

and I

OUTB

is limited to ±0.5 V. If a single-ended unipolar output

is desirable, I

OUTA

should be selected.

The distortion and noise performance of the AD9709 can be

enhanced when it is configured for differential operation. The

common-mode error sources of both I

OUTA

and I

OUTB

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. 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 (that is, assuming no

source termination). Because the output currents of I

OUTA

and

OUTB

are complementary, they become additive when processed

differentially. A properly selected transformer allows the AD9709

to provide the required power and voltage levels to different loads.

The output impedance of I

OUTA

and I

OUTB

is determined by the

equivalent parallel combination of the PMOS switches

associated with the current sources and is typically 100 kΩ in

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

voltage (that is, V

OUTA

and V

OUTB

) due to the nature of a PMOS

device. As a result, maintaining I

OUTA

and/or I

OUTB

at a virtual

ground via an I-V op amp configuration results in the optimum

dc linearity. Note that the INL/DNL specifications for the

AD9709 are measured with I

OUTA

maintained at a virtual ground

via an op amp.

OUTA

and I

OUTB

also have a negative and positive voltage

compliance range that must be adhered to in order to achieve

optimum performance. The negative output compliance range

of −1.0 V is set by the breakdown limits of the CMOS process.

Operation beyond this maximum limit may result in a

breakdown of the output stage and affect the reliability of the

AD9709.

The positive output compliance range is slightly dependent on

the full-scale output current, I

OUTFS

. When I

OUTFS

is decreased

from 20 mA to 2 mA, the positive output compliance range

degrades slightly from its nominal 1.25 V to 1.00 V. The optimum

distortion performance for a single-ended or differential output

is achieved when the maximum full-scale signal at I

OUTA

and I

OUTB

does not exceed 0.5 V. Applications requiring the AD9709 output

(that is, V

OUTA

and/or V

OUTB

) to extend its output compliance range

should size R

LOAD

accordingly. Operation beyond this compliance

range adversely affects the linearity performance of the AD9709

and subsequently degrade its distortion performance.