AD8337_07 Datasheet, PDF (20/24 Pages) Analog Devices – General-Purpose, Low Cost, DC-Coupled VGA

AD8337
APPLICATIONS
PREAMPLIFIER CONNECTIONS
Noninverting Gain Configuration
The AD8337 preamplifier is an uncommitted, current-feedback
op amp that is stable for values of RFB2 ≥ 100 Ω. See Figure 66
for the noninverting feedback connections.
INPP
3
PREAMPLIFIER
+
R_G
INPN
4
–
PRAO
5
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 preamp gains ≥2, select a value of RFB2 ≥ 100 Ω and
RFB1 ≤ 100 Ω. Higher values of RFB2 reduce the bandwidth and
increase the offset voltage, but smaller values compromise
stability. If RFB1 ≤ 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. Position the pre-
amp gain resistors, RFB1 and RFB2, as close as possible to Pin 4,
INPN, to minimize stray capacitance.
R_FB1
INPP
3
INPN
4
PRAO
5
R_FB2
PREAMPLIFIER
+
–
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 VGAIN = 0 V.
25
V_GAIN = 0V
C_L = 0pF
C_L = 10pF
20 C_L = 22pF
NO SNUBBING RESISTOR
15
10
5
0
–5
100k
1M
10M
100M
500M
FREQUENCY (Hz)
Figure 68. Peaking in the Frequency Response for Two Values of Output
Capacitance with ±2.5 V Supplies and No Snubbing Resistor
25
V_GAIN = 0V
C_L = 0pF
C_L = 10pF
20 C_L = 22pF
WITH 20Ω SNUBBING RESISTOR
15
10
5
0
–5
100k
1M
10M
FREQUENCY (Hz)
100M
500M
Figure 69. Frequency Response for Two Values of Output Capacitance
with a 20 Ω Snubbing Resistor
Rev. B | Page 20 of 24