OPA690 datasheet(16/33 Pages) TI2

16 / 33 page

OPA699

SBOS261D

www.ti.com

WIDEBAND INVERTING OPERATION

Operating the OPA699 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 –4V/V circuit used as the basis of the

inverting mode typical characteristics.

LOW-GAIN COMPENSATION FOR IMPROVED SFDR

Where a low gain is desired, and inverting operation is

acceptable, a new external compensation technique can be

used to retain the full slew rate and noise benefits of the

OPA699, while giving increased loop gain and the associ-

ated distortion improvements offered by a non-unity-gain

stable op amp. This technique shapes the loop gain for good

stability, while giving an easily controlled 2nd-order low-pass

frequency response. To set the compensation capacitors (CS

and CF), consider the half-circuit of Figure 5, where the 50Ω

source is used.

Considering only the noise gain for the circuit of Figure 5, the

low-frequency noise gain (NG1) is set by the resistor ratio,

while the high-frequency noise gain (NG2) is set by the

capacitor ratio. The capacitor values set both the transition

frequencies and the high-frequency noise gain. If the high-

frequency noise gain, determined by NG2 = 1 + CS/CF, is set

to a value greater than the recommended minimum stable

gain for the op amp, and the noise gain pole (set by 1/RFCF)

is placed correctly, a very well controlled 2nd-order low-pass

frequency response results.

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 329

Ω in this inverting configuration. The gain

resistor is set to get the desired gain (in this case, 187

Ω for

a gain of –4) 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 = 68.1Ω in parallel with the 187Ω

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 input

requires a 169

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 cancella-

tion 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.

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 –15

(RG = 50Ω, RM = open, RF = 750Ω) the noise gain for the

circuit of Figure 3 will be 1 + 750

Ω/(50Ω + 50Ω) = 8.5 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 +15. Using the 1GHz gain bandwidth

product for the OPA699, an inverting gain of –15 from a 50

Ω

source to a 50

Ω R

G will give 140MHz bandwidth, whereas

the noninverting gain of +8 will give 55MHz, as shown in the

measured results of Figure 4.

OPA699

–5V

–2V

+5V

+2V

68.1

Ω

750

Ω

187

Ω

500

Ω

0.1

µF

169

Ω

Ω Source

10M

100M

Frequency (Hz)

G = +15

G = –15

402

Ω

13pF

OPA699

+5V

–5V

2.8pF

200

Ω

402

Ω

FIGURE 3. Inverting G = –4 Specifications and Test Circuit.

FIGURE 4. G = +15 and –15 Frequency Response.

FIGURE 5. Broadband, Low-Inverting Gain External

Compensation.

OPA690 Datasheet(PDF) 16 Page - Texas Instruments

OPA690 Datasheet(HTML) 16 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

Datasheet Download

Link URL