OPA842IDBVT datasheet(12/19 Pages) TI

12 / 19 page

OPA842

SBOS267A

www.ti.com

THREE OP AMP DIFFERENCING

(Instrumentation Topology)

The primary drawback of the single op amp differential

amplifier is its relatively low input impedances. Where high

impedance is required at the differential input, a standard

instrumentation amplifier (INA) topology may be built using

the OPA842 as the differencing stage. Figure 4 shows an

example of this, in which the two input amplifiers are pack-

aged together as a dual voltage-feedback op amp, the

OPA2822. This approach saves board space, cost, and

power compared to using two additional OPA842 devices,

and still achieves very good noise and distortion perfor-

mance due to the moderate loading on the input amplifiers.

requires its outputs terminated to a compliance voltage other

than ground for operation, then the appropriate voltage level

may be applied to the noninverting input of the OPA842.

FIGURE 4. Wideband 3-Op Amp Differencing Amplifier.

OPA842

Power-supply decoupling not shown.

500

Ω

OPA2822

+5V

–5V

OPA2822

500

Ω

500

Ω

500

Ω

500

Ω

500

Ω

500

Ω

In this circuit, the common-mode gain to the output is always

1, due to the four matched 500

Ω resistors, whereas the

differential gain is set by (1 + 2RF1/RG), which is equal to 2

using the values in Figure 4. The differential to single-ended

conversion is still performed by the OPA842 output stage.

The high-impedance inputs allow the V1 and V2 sources to be

terminated or impedance matched as required. If the V1 and

V2 inputs are already truly differential, such as the output

from a signal transformer, then a single matching termination

resistor may be used between them. Remember, however,

that a defined DC signal path must always exist for the V1

and V2 inputs; for the transformer case, a center-tapped

secondary connected to ground would provide an optimum

DC operating point.

DAC TRANSIMPEDANCE AMPLIFIER

High-frequency Digital-to-Analog Converters (DACs) require

a low-distortion output amplifier to retain their SFDR perfor-

mance into real-world loads. A single-ended output drive

implementation is shown in Figure 5. 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 OPA842, 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

FIGURE 5. Wideband Low-Distortion DAC Transimpedance

Amplifier.

OPA842

High-Speed

DAC

O = ID RF

GBP

→ Gain Bandwidth

Product (Hz) for the OPA842

The DC gain for this circuit is equal to RF. At high frequen-

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

noise gain for the OPA842 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 feed-

back network should be set to:

ππ

GBP

(1)

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

GBP

−

(2)

ACTIVE FILTERS

Most active filter topologies will have exceptional performance

using the broad bandwidth and unity-gain stability of the

OPA842. Topologies employing capacitive feedback require a

unity-gain stable, voltage-feedback op amp. Sallen-Key filters

simply use the op amp as a noninverting gain stage inside an

RC network. Either current- or voltage-feedback op amps may

be used in Sallen-Key implementations.

See Figure 6 for an example Sallen-Key low-pass filter, in

which the OPA842 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 bandwidth provided

by the OPA842 in this configuration. This filter may be com-

bined with the ADC driver suggestions to provide moderate (2-

pole) Nyquist filtering, limiting noise, and out-of-band harmon-

ics into the input of an ADC. This filter will deliver the

exceptionally low harmonic distortion required by high SFDR

ADCs such as the ADS850 (14-bit, 10MSPS, 82dB SFDR).

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