DAC902 datasheet(13/25 Pages) TI2

13 / 25 page

DAC902

SBAS094B

As shown in Figure 3, the transformer’s center tap is con-

nected to ground. This forces the voltage swing on IOUT and

IOUT to be centered at 0V. In this case the two resistors, RS,

may be replaced with one, RDIFF, or omitted altogether. This

approach should only be used if all components are close to

each other, and if the VSWR is not important. A complete

power transfer from the DAC output to the load can be

realized, but the output compliance range should be ob-

served. Alternatively, if the center tap is not connected, the

signal swing will be centered at RS • IOUTFS/2. However, in

this case, the two resistors (RS) must be used to enable the

necessary DC-current flow for both outputs.

The OPA680 is configured for a gain of two. Therefore,

operating the DAC902 with a 20mA full-scale output will

produce a voltage output of

±1V. This requires the amplifier

to operate off of a dual power supply (

±5V). The tolerance

of the resistors typically sets the limit for the achievable

common-mode rejection. An improvement can be obtained

by fine tuning resistor R4.

This configuration typically delivers a lower level of ac

performance than the previously discussed transformer solu-

tion because the amplifier introduces another source of dis-

tortion. Suitable amplifiers should be selected based on their

slew-rate, harmonic distortion, and output swing capabilities.

High-speed amplifiers like the OPA680 or OPA687 may be

considered. The ac performance of this circuit may be im-

proved by adding a small capacitor, CDIFF, between the

outputs IOUT and IOUT (as shown in Figure 4). This will

introduce a real pole to create a low-pass filter in order to

slew-limiting the DACs fast output signal steps that other-

wise could drive the amplifier into slew-limitations or into an

overload condition; both would cause excessive distortion.

The difference amplifier can easily be modified to add a level

shift for applications requiring the single-ended output volt-

age to be unipolar, i.e., swing between 0V and +2V.

DUAL TRANSIMPEDANCE OUTPUT CONFIGURATION

The circuit example of Figure 5 shows the signal output

currents connected into the summing junction of the

OPA2680, which is set up as a transimpedance stage, or

I-to-V converter. With this circuit, the DAC’s output will be

kept at a virtual ground, minimizing the effects of output

impedance variations, which results in the best DC linearity

(INL). However, as mentioned previously, the amplifier

may be driven into slew-rate limitations, and produce un-

wanted distortion. This may occur especially at high DAC

update rates.

DIFFERENTIAL CONFIGURATION USING AN OP AMP

If the application requires a DC-coupled output, a difference

amplifier may be considered, as shown in Figure 4. Four

external resistors are needed to configure the voltage-feed-

back op amp OPA680 as a difference amplifier performing

the differential to single-ended conversion. Under the shown

configuration, the DAC902 generates a differential output

signal of 0.5Vp-p at the load resistors, RL. The resistor

values shown were selected to result in a symmetric 25

Ω

loading for each of the current outputs since the input

impedance of the difference amplifier is in parallel to resis-

tors RL, and should be considered.

FIGURE 4. Difference Amplifier Provides Differential to

Single-Ended Conversion and DC-Coupling.

FIGURE 5. Dual, Voltage-Feedback Amplifier OPA2680

Forms Differential Transimpedance Amplifier.

1/2

OPA2680

1/2

OPA2680

DAC902

–V

OUT = IOUT • RF

–V

OUT = IOUT • RF

OUT

Ω

–5V

+5V

OUT

DAC902

26.1

Ω

28.7

Ω

402

Ω

200

Ω

402

Ω

200

Ω

OPA680

OPT

+5V

OUT

–5V

DAC902

OUT

1:1

ADT1-1WT

(Mini-Circuits)

Ω

Optional

DIFF

FIGURE 3. Differential Output Configuration Using an RF

Transformer.

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