OPA698 datasheet(16/27 Pages) TI

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OPA698

SBOS258B

www.ti.com

WIDEBAND INVERTING OPERATION

Operating the OPA698 as an inverting amplifier has several

benefits and is particularly useful when a matched 50

Ω

source and input impedance are required. Figure 3 shows

the inverting gain of –2 circuit used as the basis of the

inverting mode typical characteristics.

As the required RG resistor approaches 50Ω at higher gains,

the bandwidth for the circuit in Figure 3 will far exceed the

bandwidth at that same gain magnitude for the noninverting

circuit of Figure 1. This occurs due to the lower noise gain for

the circuit of Figure 3 when the 50

Ω source impedance is

included in the analysis. For instance, at a signal gain of –8

(RG = 50Ω, RM = open, RF = 402Ω) the noise gain for the

circuit of Figure 3 will be 1 + 402

Ω/(50Ω + 50Ω) = 5 due to

the addition of the 50

Ω source in the noise gain equation.

This approach gives considerably higher bandwidth than the

noninverting gain of +8. Using the 250MHz gain bandwidth

product for the OPA698, an inverting gain of –8 from a 50

Ω

source to a 50

Ω R

G will give 52MHz bandwidth, whereas

the noninverting gain of +8 will give 28MHz, as shown in

Figure 4.

10k

100k

Frequency (MHz)

G = +8

G = –8

OPA698

S = +5V

+3.5V

+1.5V

REFB

REFT

0.1

µF

100pF

H = +3.6V

L = +1.4V

0.1

µF

0.1

µF

0.1

µF

402

Ω

24.9

Ω

562

Ω

102

Ω

402

Ω

715

Ω

715

Ω

102

Ω

562

Ω

ADS822

10-Bit

40MSPS

10-Bit

Data

S = +5V

INT/EXT

RSEL

GND

LIMITED OUTPUT, ADC INPUT DRIVER

Figure 5 shows a simple ADC driver that operates on a single

supply, and gives excellent distortion performance. The limit

voltages track the input range of the converter, completely

protecting against input overdrive. Note that the limiting

voltages have been set 100mV above/below the correspond-

ing reference voltage from the converter.

OPA698

–5V

–2V

+5V

+2V

66.5

Ω

402

Ω

200

Ω

500

Ω

0.1

µF

147

Ω

Ω Source

In the inverting case, only the feedback resistor appears as

part of the total output load in parallel with the actual load.

For a 500

Ω load used in the typical characteristics, this gives

a total load of 222

Ω in this inverting configuration. The gain

resistor is set to get the desired gain (in this case, 200

Ω for

a gain of –2) while an additional input resistor (RM) can be

used to set the total input impedance equal to the source, if

desired. In this case, RM = 66.5Ω in parallel with the 200Ω

gain setting resistor gives a matched input impedance of

Ω. This matching is only needed when the input needs to

be matched to a source impedance, as in the characteriza-

tion testing done using the circuit of Figure 3.

For bias current-cancellation matching, the noninverting in-

put requires a 147

Ω resistor to ground. The calculation for

this resistor includes a DC-coupled 50

Ω source impedance

along with RG and RM. Although this resistor will provide

cancellation for the bias current, it must be well-decoupled

(0.1

µF in Figure 3) to filter the noise contribution of the

resistor and the input current noise.

FIGURE 3. Inverting G = –2 Specifications and Test Circuit.

FIGURE 4. G = +8 and –8 Frequency Response.

FIGURE 5. Single Supply, Limiting ADC Input Driver.

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