AD8310ARMZ-REEL71 AD [Analog Devices], AD8310ARMZ-REEL71 Datasheet - Page 18

• Manufacturer Part Number: AD8310ARMZ-REEL71
• Description: Fast, Voltage-Out DC-440 MHz, 95 dB Logarithmic Amplifier
• Manufacturer: AD [Analog Devices]

AD8310
OUTPUT FILTERING
For applications in which maximum video bandwidth and,
consequently, fast rise time are desired, it is essential that the
BFIN pin be left unconnected and free of any stray capacitance.
The nominal output video bandwidth of 25 MHz can be
reduced by connecting a ground-referenced capacitor (C
the BFIN pin, as shown in Figure 35. This is generally done to
reduce output ripple (at twice the input frequency for a
symmetric input waveform such as sinusoidal signals).
C
The video bandwidth should typically be set at a frequency
equal to about one-tenth the minimum input frequency. This
ensures that the output ripple of the demodulated log output,
which is at twice the input frequency, is well filtered.
In many log amp applications, it might be necessary to lower the
corner frequency of the postdemodulation filtering to achieve
low output ripple while maintaining a rapid response time to
changes in signal level. An example of a 4-pole active filter is
shown in the AD8307 data sheet.
FILT
is selected using the following equation:
C
Figure 35. Lowering the Postdemodulation Video Bandwidth
FILT
C
FILT
=
(
= 1/(2π × 3kΩ × VIDEO BANDWIDTH) – 2.1pF
2µA/dB
2
π
×
3kΩ
3
k
Ω
×
VideoBandw
AD8310
1
+4
BFIN
V
OUT
idth
)
C
−
FILT
2
1 .
pF
FILT
) to
Rev. D | Page 18 of 24
LOWERING THE HIGH-PASS CORNER FREQUENCY
OF THE OFFSET COMPENSATION LOOP
In normal operation using an ac-coupled input signal, the OFLT
pin should be left unconnected. Input-referred dc offsets of
about 1.5 mV in the signal path are nulled via an internal offset
control loop. This loop has a high-pass −3 dB corner at about
2 MHz. In low frequency ac-coupled applications, it is necessary
to lower this corner frequency to prevent input signals from
being misinterpreted as offsets. An external capacitor on OFLT
lowers the high-pass corner to arbitrarily low frequencies
(Figure 36). For example, by using a 1 µF capacitor, the 3 dB
corner is reduced to 60 Hz.
The corner frequency is set by the following equation:
where C
f
CORNER
OFLT
Figure 36. Lowering the High-Pass Corner Frequency
is the capacitor connected to OFLT.
=
(
2
π
of the Offset Control Loop
×
AD8310
2625
1
×
OFLT
C
OFLT
C
(SEE TEXT)
OFLT
)