AD8319

Rev. A | Page 15 of 20

OUTPUT FILTERING

OPERATION BEYOND 8 GHZ

The AD8319 is specified for operation up to 8 GHz, but it provides

useful measurement accuracy over a reduced dynamic range of

up to 10 GHz. Figure 35 shows the performance of the AD8319

over temperature at 10 GHz when the device is configured as

shown in Figure 22. Dynamic range is reduced at this frequency,

but the AD8319 does provide 30 dB of measurement range within

±3 dB of linearity error.

For applications in which maximum video bandwidth and,

consequently, fast rise time are desired, it is essential that the

CLPF pin be left unconnected and free of any stray capacitance.

The nominal output video bandwidth of 50 MHz can be

the CLPF pin, as shown in Figure 34. This is generally done to

reduce output ripple (at twice the input frequency for a

symmetric input waveform such as sinusoidal signals).

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

–40

–35

–30

–25

–20

–15

–10

–5

0

5

–5

–4

–3

–2

–1

0

1

2

3

4

5

+4

VOUT

CLPF

AD8319

3.5pF

1.5kΩ

Figure 34. Lowering the Postdemodulation Bandwidth

()

pF

3.5

kΩ

1.5

π

1

−

×

×

=

Bandwidth

Video

C

FLT

(11)

The video bandwidth should typically be set to 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.

Implementing an impedance match for frequencies beyond

8 GHz can improve the sensitivity of the AD8319 and

measurement range.

Operation beyond 10 GHz is possible, but part-to-part

variation, most notably in the intercept, becomes significant.

In many log amp applications, it may 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.