AD8310ARM datasheet(18/24 Pages) AD

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AD8310

Rev. D | 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 output ripple (at twice the input frequency for a

symmetric input waveform such as sinusoidal signals).

µA/dB

Ω

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

()

−

Ω

idth

VideoBandw

CFILT

The corner frequency is set by the following equation:

()

OFLT

CORNER

2625

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.

where COFLT is the capacitor connected to OFLT.

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.

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