DAC900 datasheet(13/25 Pages) TI2

13 / 25 page

DAC900

SBAS093B

FIGURE 3. Differential Output Configuration Using an RF

Transformer.

FIGURE 4. Difference Amplifier Provides Differential to

Single-Ended Conversion and DC-Coupling.

FIGURE 5. Dual, Voltage-Feedback Amplifier OPA2680

Forms Differential Transimpedance Amplifier.

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.

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 DAC900 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.

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

operating the DAC900 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

distortion. Suitable amplifiers should be selected based on

their slew-rate, harmonic distortion, and output swing capa-

bilities. High-speed amplifiers like the OPA680 or OPA687

may be considered. The ac performance of this circuit may

be improved by adding a small capacitor, CDIFF, between the

outputs IOUT and IOUT, as shown in Figure 4. This will intro-

duce a real pole to create a low-pass filter in order to slew-

limit the DACs fast output signal steps that otherwise could

drive the amplifier into slew-limitations or into an overload

condition; both would cause excessive distortion. The differ-

ence amplifier can easily be modified to add a level shift for

applications requiring the single-ended output voltage 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, and resulting 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.

OUT

DAC900

1:1

ADT1-1WT

(Mini-Circuits)

Ω

Optional

DIFF

OUT

DAC900

26.1

Ω

28.7

Ω

402

Ω

200

Ω

402

Ω

200

Ω

OPA680

DIFF

+5V

OUT

–5V

1/2

OPA2680

1/2

OPA2680

DAC900

–V

OUT = IOUT • RF

–V

OUT = IOUT • RF

OUT

Ω

–5V

+5V

DAC900 Datasheet(PDF) 13 Page - Texas Instruments

DAC900 Datasheet(HTML) 13 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Datasheet Download

Link URL