10

®

OPA642

DAC TRANSIMPEDANCE AMPLIFIER

High frequency DDC DACs require a low distortion output

amplifier to retain their SFDR performance into real-world

loads. A single-ended output drive implementation is shown

in Figure 4. In this circuit, only one side of the complemen-

tary output drive signal is used. The diagram shows the

signal output current connected into the virtual ground

summing junction of the OPA642, which is set up as a

transimpedance stage or “I-V converter”. The unused cur-

rent output of the DAC is connected to ground. If the DAC

requires its outputs terminated to a compliance voltage other

than ground for operation, then the appropriate voltage level

may be applied to the non-inverting input of the OPA642.

cies, the DAC output capacitance will produce a zero in the

noise gain for the OPA642 that may cause peaking in the

compensate for this noise gain peaking. To achieve a flat

transimpedance frequency response, this pole in the feed-

back network should be set to:

Figure 5 shows an example Sallen-Key low pass filter, in

which the OPA642 is set up to deliver a low frequency gain

of +2. The filter component values have been selected to

achieve a maximally flat Butterworth response with a 5MHz

–3dB bandwidth. The resistor values have been slightly

adjusted to compensate for the effects of the 150MHz band-

width provided by the OPA642 in this configuration. This

filter may be combined with the ADC driver suggestions to

provide moderate (2-pole) Nyquist filtering, limiting noise

and out of band components into the input of an ADC. This

filter will deliver the exceptionally low harmonic distortion

required by high SFDR A/D converters such as the ADS804

(12-bit, 10MSPS, 80dB SFDR).

OPA642

High Speed

DAC

V

R

F

C

F

GBP

→ Gain Bandwidth

Product for the OPA642

C

D

I

O

I

O

FIGURE 4. Wideband, Low Distortion DAC Transimpedance

Amplifier.

ACTIVE FILTERS

Most active filter topologies will deliver exceptional perfor-

mance using the broad bandwidth and unity gain stability of

the OPA642. Topologies employing capacitive feedback

require a unity gain stable voltage feedback op amp. Sallen-

Key filters simply use the op amp as a non-inverting gain

stage inside an RC network. Either current or voltage feed-

back op amps may be used in Sallen-Key implementations.

FIGURE 5. 5MHz Butterworth Low Pass Active Filter.

V

O

402

Ω

505

Ω

V

I

124

Ω

OPA642

100pF

402

Ω

150pF

OPERATING SUGGESTIONS

OPTIMIZING RESISTOR VALUES

Since the OPA642 is a unity gain stable voltage feedback op

amp, a wide range of resistor values may be used for the

feedback and gain setting resistors. The primary limits on

these values are set by dynamic range (noise and distortion)

and parasitic capacitance considerations. For a non-inverting

unity gain follower application, the feedback connection

should be made with a 25

Ω resistor—not a direct short. This

will isolate the inverting input capacitance from the output

pin and improve the frequency response flatness. Usually,

the feedback resistor value should be between 200

Ω and

1k

Ω. Below 200Ω, the feedback network will present

additional output loading which can degrade the harmonic

distortion performance of the OPA642. Above 1k

Ω, the

typical parasitic capacitance (approximately 0.2pF) across

the feedback resistor may cause unintentional band-limiting

in the amplifier response.

A good rule of thumb is to target the parallel combination of

input capacitance, placing an additional pole in the feedback

network and thus a zero in the forward response. Assuming

< 200

Ω will keep this pole above 400MHz. By itself, this

to several k

Ω at high gains. This is acceptable as long as the

in parallel is kept out of the frequency range of interest.

ƒ

GBP / 2

πR

1/ 2

πR

GBP / 4

πR

()