AD9705BCPZ datasheet(35/52 Pages) AD

35 / 52 page

AD9704/AD9705/AD9706/AD9707

Rev. 0 | Page 35 of 52

Also note that the full-scale value of VIOUTA and VIOUTB 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 – (2N − 1))/2N} ×

(8)

(32 × VREFIO/RSET) × RLOAD

Equation 7 and Equation 8 highlight some of the advantages of

operating the AD970x 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, VIOUTA or VIOUTB), thus providing twice the signal power

to the load.

Note that the gain drift temperature performance for a single-

ended output (VIOUTA and VIOUTB) or differential output (VDIFF)

of the AD970x can be enhanced by selecting temperature

tracking resistors for RLOAD and RSET, because of 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 opera-

tion. IOUTA and IOUTB can be converted into complementary

single-ended voltage outputs, VIOUTA and VIOUTB, 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 VIOUTA and VIOUTB, can also be

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

differential amplifier configuration. The ac performance of the

AD970x is optimum and is 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 AD970x 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 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 (assuming no source

termination). Because the output currents of IOUTA and

IOUTB are complementary, they become additive when

processed differentially.

When the AD970x is being used at its nominal operating point

of 2 mA output current, and 0.5 V output swing is desired,

RLOAD must be set to 250 Ω. A properly selected transformer

allows the AD970x 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 200 MΩ in

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

voltage (that is, VIOUTA and VIOUTB) 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 results in the

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

the AD970x are measured with IOUTA maintained at a virtual

ground via an op amp.

IOUTA and IOUTB also have a negative and positive voltage

compliance range that must be adhered to in order to achieve

optimum performance. The absolute maximum negative output

compliance range of −1 V is set by the breakdown limits of the

CMOS process. Operation beyond this maximum limit can result

in a breakdown of the output stage and affect the reliability of

the AD970x.

The positive output compliance range is slightly dependent on

the full-scale output current, IOUTFS. It degrades slightly from its

nominal 1.0 V for an IOUTFS = 2 mA to 0.8 V for an IOUTFS = 1 mA.

The optimum distortion performance for a single-ended or

differential output is achieved when the maximum full-scale

signal at IOUTA and IOUTB does not exceed 0.5 V.

ADJUSTABLE OUTPUT COMMON MODE

The AD970x provides the ability to set the output common

mode to a value other than ACOM via Pin 19 (OTCM). This

extends the compliance range of the outputs and facilitates

interfacing the output of the AD970x to components that

require common-mode levels other than 0 V. The OTCM pin

demands dynamically changing current and should be driven

by a low source impedance to prevent a common-mode signal

from appearing on the DAC outputs. For optimum performance,

set the voltage on OTCM equal to the center of the output

swing on IOUTA and IOUTB.

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