DAC2902

12

SBAS167C

www.ti.com

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

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

will produce a 0 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 response, the

pole in each feedback network should be set to:

1

24

ππ

RC

GBP

RC

FF

F

D

=

(8)

with GBP = Gain Bandwidth Product of OPA,

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

f

GBP

RC

dB

FD

−

=

3

2

π

(9)

The full-scale output voltage is simply defined by the prod-

uct 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 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 mutu-

ally adjusted to provide the desired output signal swing and

performance.

INTERFACING ANALOG

QUADRATURE MODULATORS

One of the main applications for the dual-channel DAC is

baseband I- and Q-channel transmission for digital commu-

nications. In this application, the DAC is followed by an

analog quadrature modulator, modulating an IF carrier with

the baseband data, as shown in Figure 7. Often, the input

stages of these quadrate modulators consist of npn-type

transistors that require a DC bias (base) voltage of > 0.8V.

The wide output compliance range (–10V to +1.25V) allows

for a direct DC–coupling between the DAC2902 and the

quadrature modulator.

FIGURE 6. Driving a Doubly Terminated 50

Ω Cable Directly.

FIGURE 7. Generic Interface to a Quadrature Modulator. Signal Conditioning (Level-Shifting) May Be Required to Ensure

Correct DC Common-Mode Levels At the Input of the Quadrature Modulator.

I

OUT

I

OUT

DAC2902

25

Ω

50

Ω

50

Ω

I

OUTFS = 20mA

V

OUT = 0V to +0.5V

I

OUT1

I

OUT1

I

OUT2

I

OUT2

DAC2902

Signal

Conditioning

I

IN

I

REF

Q

IN

Q

REF

∑

Quadrature Modulator

V

OUT ~ 0Vp to 1.20Vp

V

IN ~ 0.6Vp to 1.8Vp

RF

I

IN

I

REF