ADA4860-1

Rev. 0 | Page 16 of 20

OPTIMIZING FLATNESS AND BANDWIDTH

When using the ADA4860-1, a variety of circuit conditions and

parasitics can affect peaking, gain flatness, and −3 dB

bandwidth. This section discusses how the ADA4860-1 small

signal responses can be dramatically altered with basic circuit

changes and added stray capacitances, see the Layout and

Circuit Board Parasitics section for more information.

Particularly with low closed-loop gains, the feedback resistor

−3 dB bandwidth varies slightly, while gain = +2 has a much

larger variation. For gain = +1, Figure 56 shows the effect that

various feedback resistors have on frequency response. In

Figure 56, peaking is wide ranging yet −3 dB bandwidths vary

by only 6%. In this case, the user must pick what is desired:

more peaking or flatter bandwidth. Figure 57 shows gain = +2

from 560 Ω to 301 Ω, the −3 dB bandwidth changes 49%, with

excessive compromises in peaking, see Figure 57. For more gain

2

–6

–5

–4

–3

–2

–1

0

1

1

100

10

10000

1000

FREQUENCY (MHz)

G = +1

2

–6

–5

–4

–3

–2

–1

0

1

1

100

10

1000

FREQUENCY (MHz)

G = +2

The impact of resistor case sizes was observed using the circuit

drawn in Figure 58. The types and sizes chosen were 0402 case

sized thin film and 1206 thick film. All other measurement

conditions were kept constant except for the case size and

resistor composition.

EXAMPLE

49.9Ω

49.9Ω

+

–

50Ω

EXAMPLE

DASH LINE IS PLANE CLEAR OUT AREA

(EXCEPT SUPPLY PINS) DURING PC LAYOUT.

Figure 58. Noninverting Gain Setup for Illustration of

In Figure 59, a slight −3 dB bandwidth delta of approximately

+10% can be seen going from a small-to-large case size. The

increase in bandwidth with the larger 1206 case size is caused

by an increase in parasitic capacitance across the chip resistor.

1

–6

–5

–4

–3

–2

–1

0

1

100

10

1000

FREQUENCY (MHz)

G = +2

1206 RESISTOR SIZE

0402 RESISTOR SIZE

Figure 59. Small Signal Frequency Response vs. Resistor Size