OPA699M datasheet(19/32 Pages) TI

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10M

100M

Frequency (Hz)

G = +15

G = -15

G047

LOW-GAIN COMPENSATION FOR IMPROVED SFDR

OPA699M

SGLS289A – MAY 2005 – REVISED MARCH 2006

APPLICATION INFORMATION (continued)

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 = 68.1 Ω in parallel with the 187-Ω gain setting resistor gives a matched input impedance of 50 Ω. This

matching is only needed when the input needs to be matched to a source impedance, as in the characterization

testing done using the circuit of Figure 49.

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 R

G and RM. Although this

resistor provides cancellation for the bias current, it must be well-decoupled (0.1 µF in Figure 49) to filter the

noise contribution of the resistor and the input current noise.

As the required RG resistor approaches 50 Ω at higher gains, the bandwidth for the circuit in Figure 49 far

exceeds the bandwidth at that same gain magnitude for the noninverting circuit of Figure 47. This occurs due to

the lower noise gain for the circuit of Figure 49 when the 50-

Ω source impedance is included in the analysis. For

instance, at a signal gain of –15 (RG = 50 Ω, RM = open, RF = 750 Ω) the noise gain for the circuit of Figure 49

will be 1 + 750

Ω/(50 Ω + 50 Ω) = 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 1-GHz gain

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

Ω source to a 50-Ω RG gives 140-MHz

bandwidth, whereas the noninverting gain of +6 gives 55 MHz, as shown in the measured results of Figure 50.

Figure 50. G = +15 and -15 Frequency Response

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

associated 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 51, where the 50-Ω source is used.

Considering only the noise gain for the circuit of Figure 51, 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 well controlled 2

nd-order low-pass frequency response

results.

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