Datasheet

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
(R
f
) effects peaking and gain flatness. However, with gain = +1,
−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
bandwidth and peaking variations vs. R
F
and R
L
. Bandwidth
delta vs. R
L
increase was approximately 17%. As R
F
is reduced
from 560 Ω to 301 Ω, the −3 dB bandwidth changes 49%, with
excessive compromises in peaking, see
Figure 57. For more gain
= +2 bandwidth variations vs. R
F
, see Figure 10 and Figure 13.
2
–6
–5
–4
–3
–2
–1
0
1
1 10010 100001000
NORMALIZED GAIN (dB)
FREQUENCY (MHz)
05709-044
V
S
= ±5V
G = +1
V
OUT
= 0.1V p-p
R
L
= 100Ω
R
F
= 560Ω
R
F
= 680Ω
R
F
= 910Ω
R
F
= 1.5kΩ
Figure 56. Small Signal Frequency Response vs. R
F
2
–6
–5
–4
–3
–2
–1
0
1
1 10010 1000
NORMALIZED GAIN (dB)
FREQUENCY (MHz)
05709-045
V
S
= ±5V
G = +2
V
OUT
= 0.1V p-p
R
G
= R
F
R
F
= 301Ω, R
L
= 100Ω
R
F
= 560Ω, R
L
= 100Ω
R
F
= 560Ω, R
L
= 1kΩ
Figure 57. Small Signal Frequency Response vs. R
F
vs. R
L
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.
05709-049
ADDED C
LOAD
EXAMPLE
R
G
49.9Ω
49.9Ω
+
–
50Ω
R
F
ADDED C
J
EXAMPLE
DASH LINE IS PLANE CLEAR OUT ARE
A
(EXCEPT SUPPLY PINS) DURING PC LAYOUT.
Figure 58. Noninverting Gain Setup for Illustration of
Parasitic Effects, 50 Ω System, R
L
= 100 Ω
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 10010 1000
NORMALIZED GAIN (dB)
FREQUENCY (MHz)
05709-046
V
S
= ±5V
G = +2
V
OUT
= 0.1V p-p
R
G
= R
F
= 560Ω
R
L
= 100Ω
1206 RESISTOR SIZE
0402 RESISTOR SIZE
Figure 59. Small Signal Frequency Response vs. Resistor Size