OPA820SKGD3 datasheet(21/30 Pages) TI1

21 / 30 page

O P A820

High−Speed

DAC

O = ID

GBP → Gain Bandwidth

Product (Hz) for the OPA820.

GBP

2 R C

4 R C

3dB

GBP

4 R C

OPA820-HT

www.ti.com

SBOS587

– DECEMBER 2011

DAC Transimpedance Amplifier

High-frequency digital-to-analog converters (DACs) require a low-distortion output amplifier to retain their SFDR

performance into real-world loads. See Figure 8 for a single-ended output drive implementation. In this circuit,

only one side of the complementary output drive signal is used. The diagram shows the signal output current

connected into the virtual ground-summing junction of the OPA820, which is set up as a transimpedance stage or

I-V converter. The unused current output of the DAC is connected to ground. If the DAC requires its outputs to

be 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 OPA820.

Figure 8. Wideband, Low-Distortion DAC Transimpedance Amplifier

The DC gain for this circuit is equal to RF. At high frequencies, the DAC output capacitance (CD) will produce a

zero in the noise gain for the OPA820 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, this pole in the feedback network should be set to:

(1)

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

(2)

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