AD8170/AD8174
Equation 4 can be used to calculate expected gain error due to
the current feedback amplifier’s finite transimpedance and
common mode rejection. For low gains and recommended
feedback resistors, this will be typically less than 0.4%. For
most applications with gain greater than 1, the dominant source
of gain error will most likely be the ratio-match of the external
resistors. All of the dominant contributors to gain error are
associated with the buffer amplifier and external resistors.
These do not change as different channels are selected, so
channel-to-channel gain match of less than 0.05% is easily
attained.
R
R
T
G
=
1+
F
1−
CMRR
R
F
R
G
R
T
+
R
IN
1+
+
R
F
R
G
A
CL
=
Closed Loop Gain
C
T
=
Transcapacitance 0.8 pF
R
F
= Feedback Resistor
G
= Ideal Closed Loop Gain
G
N
= (1 + R
F
/R
G
) = Noise Gain
R
IN
= Inverting Terminal Input Resistance
≅
100
Ω
The –3 dB bandwidth is determined from this model as:
f
–3
dB
≅
1
2π
C
T
(
R
F
+G
N
R
IN
)
This model is typically good to within 15%.
[
]
(4)
Table III. Recommended Component Values
Small Signal
Large Signal
V
OUT
= 50 mV rms V
OUT
= 0.707 V rms
Gain R
F
( ) R
G
( ) –3 dB BW (MHz) –3 dB BW (MHz)
AD8170R +1
+2
+10
+20
AD8174R +1
+2
+10
+20
1k
499
499
499
1k
549
499
499
—
499
54.9
26.3
—
549
54.9
26.3
710
250
50
27
780
235
50
27
270
290
55
27
270
280
55
27
↑
Ideal Gain
↑
Error Terms
R
T
= Amplifier Transresistance = 600 kΩ
R
IN
= Amplifier Input Resistance
≅
100
Ω
CMRR
= Amplifier Common-Mode Rejection
≅
–52 dB
Choice of External Resistors
The gain and bandwidth of the multiplexer are determined by
the closed-loop gain and bandwidth of the onboard current
feedback amplifier. These both may be customized by the
external resistor feedback network. Table III shows typical
bandwidths at some common closed loop gains for given
feedback and gain resistors (R
F
and R
G
, respectively).
The choice of R
F
is not critical unless the widest and flattest
frequency response must be maintained. The resistors recom-
mended in the table result in the widest 0.1 dB bandwidth with
the least peaking. 1% resistors are recommended for applications
requiring the best control of bandwidth. Packaging parasitics vary
between DIP and SOIC packages, which may result in a slightly
different resistor value for optimum frequency performance.
Wider bandwidths than those listed in the table can be attained
by reducing R
F
at the expense of increased peaking.
To estimate the –3 dB bandwidth for feedback resistors not
listed in Table III, the following single-pole model for the
current feedback amplifier may be used:
A
CL
=
G
1+
sC
T
(
R
F
+G
N
R
IN
)
Capacitive Load
The general rule for current feedback amplifiers is that the
higher the load capacitance, the higher the feedback resistor
required for stable operation. For the best combination of wide
bandwidth and clean pulse response, a small output resistor is
also recommended, as shown in Figure 24. Table IV contains
values of feedback and series resistors that result in the best
pulse response for a given load capacitance.
R
F
+V
S
R
G
0.1µF
BUFFER
V
IN
0.1µF
R
T
50Ω
SWITCH
–V
S
10µF
C
L
R
S(OUT)
V
OUT
(TO FET PROBE)
10µF
Figure 24. Circuit for Driving a Capacitive Load
Table IV. Recommended Feedback and Series Resistors and Bandwidth vs. Capacitive Load and Gain
C
L
(pF)
20
50
100
300
R
F
( )
1k
1k
2k
2k
G = +1
V
OUT
= 2 V p-p
R
SOUT
–3 dB BW
( )
(MHz)
50
30
20
20
149
104
73
27
R
F
( )
1k
1k
1k
1k
G = +2
V
OUT
= 2 V p-p
R
SOUT
–3 dB BW
( )
(MHz)
20
15
15
15
174
117
80
34
R
F
( )
499
1k
1k
1k
G = +3
V
OUT
= 2 V p-p
R
SOUT
–3 dB BW
( )
(MHz)
25
15
15
15
170
98
71
33
G
R
F
( )
499
499
499
499
+4
R
SOUT
( )
20
20
15
15
REV. 0
–9–
AD [ ANALOG DEVICES ]
