Datasheet

AD8337
Rev. C | Page 20 of 32
APPLICATIONS INFORMATION
PREAMPLIFIER CONNECTIONS
Noninverting Gain Configuration
The AD8337 preamplifier is an uncommitted current-feedback
op amp that is stable for values of R
FB2
≥ 100 Ω. See Figure 66
for the noninverting feedback connections.
R
G
PRAO
PREAMPLIFIER
+
–
INPP
INPN
5
3
4
05575-066
R
FB2
R
FB1
Figure 66. AD8337 Preamplifier Configured for Noninverting Gain
Two surface-mount resistors establish the preamplifier gain.
Equal values of 100 Ω configure the preamplifier for a 6 dB gain
and the device for a default gain range of 0 dB to 24 dB.
For preamplifier gains ≥2, select a value of R
FB2
≥ 100 Ω and
R
FB1
≤ 100 Ω. Higher values of R
FB2
reduce the bandwidth and
increase the offset voltage, but smaller values compromise
stability. If R
FB1
≤ 100 Ω, the gain increases and the input-
referred noise decreases.
Inverting Gain Configuration
For applications requiring polarity inversion of negative pulses, or
for waveforms that require current sinking, the preamplifier can
be configured as an inverting gain amplifier. When configured
with bipolar supplies, the preamplifier amplifies positive or
negative input voltages with no level shifting of the common-
mode input voltage required. Figure 67 shows the AD8337
configured for inverting gain operation.
Because the AD8337 is a very high frequency device, stability
issues can occur unless the circuit board on which it is used is
carefully laid out. The stability of the preamp is affected by
parasitic capacitance around the INPN pin. To minimize stray
capacitance position the preamp gain resistors, R
FB1
and R
FB2
, as
close as possible to the INPN pin.
PRAO
PREAMPLIFIER
+
–
INPP
INPN
5
3
4
05575-067
R
FB2
R
FB1
Figure 67. The AD8337 Preamplifier Configured for Inverting Gain
DRIVING CAPACITIVE LOADS
Because of the large bandwidth of the AD8337, stray capacitance at
the output pin can induce peaking in the frequency response as the
gain of the amplifier begins to roll off. Figure 68 shows peaking
with two values of load capacitance using ±2.5 V supplies and
V
GAIN
= 0 V.
GAIN (dB)
–5
0
10
20
15
25
100k
5
FREQUENCY (Hz)
1M 500M100M10M
05575-068
C
L
= 0pF
C
L
= 10pF
C
L
= 22pF
V
GAIN
= 0V
NO SNUBBING RESISTOR
Figure 68. Peaking in the Frequency Response for Two Values of Output
Capacitance with ±2.5 V Supplies and No Snubbing Resistor
GAIN (dB)
–5
0
10
20
15
25
100k
5
FREQUENCY (Hz)
1M 500M100M10M
05575-069
C
L
= 0pF
C
L
= 10pF
C
L
= 22pF
V
GAIN
= 0V
WITH 20Ω SNUBBING RESISTOR
Figure 69. Frequency Response for Two Values of Output Capacitance
with a 20 Ω Snubbing Resistor
In the time domain, stray capacitance at the output pin can
induce overshoot on the edges of transient signals, as shown in
Figure 70 and Figure 72. The amplitude of the overshoot is also a
function of the slewing of the transient (not shown in Figure 70
and Figure 72). The transition time of the input pulses used for
Figure 70 and Figure 72 is deliberately set high at 300 ps to demon-
strate the fast response time of the amplifier. Signals with longer
transition times generate less overshoot.