AD8310-EVALZ datasheet(18/24 Pages) AD

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AD8310

Rev. F | Page 18 of 24

OUTPUT FILTERING

LOWERING THE HIGH-PASS CORNER FREQUENCY

OF THE OFFSET COMPENSATION LOOP

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.

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 nominal output video bandwidth of 25 MHz can be reduced

by connecting a ground-referenced capacitor (CFILT) to the BFIN

pin, as shown in Figure 35. This is generally done to reduce out-

put ripple (at twice the input frequency for a symmetric input

waveform such as sinusoidal signals).

μA/dB

VOUT

BFIN

CFILT

AD8310

CFILT = 1/(2π× 3kΩ× VIDEO BANDWIDTH) – 2.1pF

()

COFLT

(SEE TEXT)

AD8310

OFLT

()

Figure 35. Lowering the Postdemodulation Video Bandwidth

CFILT is selected using the following equation:

Figure 36. Lowering the High-Pass Corner Frequency

of the Offset Control Loop

−

idth

VideoBandw

CFILT

(11)

The corner frequency is set by the following equation:

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.

OFLT

CORNER

2625

(12)

where COFLT is the capacitor connected to OFLT.

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