AD8310 datasheet(18/24 Pages) AD

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AD8310

Rev. E | Page 18 of 24

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 (CFILT) to

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).

μA/dB

VOUT

BFIN

CFILT

AD8310

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

Figure 35. Lowering the Postdemodulation Video Bandwidth

CFILT is selected using the following equation:

()

−

idth

VideoBandw

CFILT

(11)

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.

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.

COFLT

(SEE TEXT)

AD8310

OFLT

Figure 36. Lowering the High-Pass Corner Frequency

of the Offset Control Loop

The corner frequency is set by the following equation:

(

)

OFLT

CORNER

2625

(12)

where COFLT is the capacitor connected to OFLT.

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