DAC904ESLASH2K5 datasheet(13/24 Pages) TI2

13 / 24 page

DAC904

SBAS095C

www.ti.com

FIGURE 5. Dual, Voltage-Feedback Amplifier OPA2680 Forms

Differential Transimpedance Amplifier.

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 con-

verter. 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). How-

ever, as mentioned previously, the amplifier may be driven

into slew-rate limitations, and produce unwanted distortion.

This may occur especially at high DAC update rates.

The DC gain for this circuit is equal to feedback resistor RF.

At high frequencies, the DAC output impedance (CD1, CD2)

will produce a zero in the noise gain for the OPA2680 that

may cause peaking in the closed-loop frequency response.

CF is added across RF to compensate for this noise-gain

peaking. To achieve a flat transimpedance frequency re-

sponse, the pole in each feedback network should be set to:

ππ

GBP

(8)

with GBP = Gain Bandwidth Product of OPA,

which will give a corner frequency f-3dB of approximately:

GBP

−

(9)

1/2

OPA2680

1/2

OPA2680

DAC904

–V

OUT = IOUT • RF

–V

OUT = IOUT • RF

OUT

Ω

–5V

+5V

The full-scale output voltage is defined by the product of

IOUTFS • RF, and has a negative unipolar excursion. To

improve on the ac performance of this circuit, adjustment of

RF and/or IOUTFS should be considered. Further extensions of

this application example may include adding a differential

filter at the OPA2680’s output followed by a transformer, in

order to convert to a single-ended signal.

SINGLE-ENDED CONFIGURATION

Using a single load resistor connected to the one of the DAC

outputs, a simple current-to-voltage conversion can be ac-

complished. The circuit in Figure 6 shows a 50

Ω resistor

connected to IOUT, providing the termination of the further

connected 50

Ω cable. Therefore, with a nominal output

current of 20mA, the DAC produces a total signal swing of

0V to 0.5V into the 25

Ω load.

Different load resistor values may be selected as long as the

output compliance range is not exceeded. Additionally, the

output current, IOUTFS, and the load resistor may be mutually

adjusted to provide the desired output signal swing and

performance.

FIGURE 6. Driving a Doubly-Terminated 50

Ω Cable Directly.

OUT

DAC904

Ω

OUTFS = 20mA

OUT = 0V to +0.5V

INTERNAL REFERENCE OPERATION

The DAC904 has an on-chip reference circuit that comprises

a 1.24V bandgap reference and a control amplifier. Ground-

ing pin 16, INT/EXT, enables the internal reference opera-

tion. The full-scale output current, IOUTFS, of the DAC904 is

determined by the reference voltage, VREF, and the value of

resistor RSET. IOUTFS can be calculated by:

IOUTFS = 32 • IREF = 32 • VREF / RSET

(10)

The external resistor RSET connects to the FSA pin (Full-

Scale Adjust), see Figure 7. The reference control amplifier

operates as a V-to-I converter producing a reference current,

IREF, which is determined by the ratio of VREF and RSET, as

shown in Equation 10. The full-scale output current, IOUTFS,

results from multiplying IREF by a fixed factor of 32.

DAC904E/2K5 Datasheet(PDF) 13 Page - Texas Instruments

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