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AD8310ARM 数据表(PDF) 18 Page - Analog Devices |
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AD8310ARM 数据表(HTML) 18 Page - Analog Devices |
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18 / 24 page  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). +4 2 μA/dB 3k Ω 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: () pF 1 . 2 k 3 2 1 − × Ω × π = 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 C f × × π = 2625 2 1 (12) where COFLT is the capacitor connected to OFLT. |
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类似说明 - AD8310ARM |
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