DAC908 datasheet(13/24 Pages) TI2

13 / 24 page

DAC908

SBAS109B

FIGURE 6. Driving a Doubly-Terminated 50

Ω Cable Directly.

FIGURE 7. Internal Reference Configuration.

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)

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.

INTERNAL REFERENCE OPERATION

The DAC908 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 DAC908 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)

As shown in Figure 7, the external resistor RSET connects to

the FSA pin (Full-Scale Adjust). The reference control am-

plifier 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.

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.

Using the internal reference, a 2k

Ω resistor value results in a

20mA full-scale output. Resistors with a tolerance of 1% or

better should be considered. Selecting higher values, the con-

verter output can be adjusted from 20mA down to 2mA.

Operating the DAC908 at lower than 20mA output currents

may be desirable for reasons of reducing the total power

consumption, improving the distortion performance, or observ-

ing the output compliance voltage limitations for a given load

condition.

It is recommended to bypass the REFIN pin with a ceramic chip

capacitor of 0.1

µF or more. The control amplifier is internally

compensated, and its small signal bandwidth is approximately

1.3MHz. To improve the ac performance, an additional capaci-

tor (CCOMPEXT) should be applied between the BW pin and the

analog supply, +VA, as shown in Figure 7. Using a 0.1µF

capacitor, the small-signal bandwidth and output impedance of

the control amplifier is further diminished, reducing the noise

that is fed into the current source array. This also helps shunting

feedthrough signals more effectively, and improving the noise

performance of the DAC908.

OUT

DAC908

Ω

OUTFS = 20mA

OUT = 0V to +0.5V

DAC908

COMPEXT

0.1

µF

COMP

400pF

+1.24V Ref.

SET

Ω

0.1

µF

INT/EXT

FSA

+5V

REF

Current

Sources

REF =

REF

SET

Ref

Control

Amp

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