AD8307AN datasheet(23/24 Pages) AD

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AD8307

Rev. C | Page 23 of 24

The intercept can be raised, for example, to 100 μV, with the

rationale that the dc precision does not warrant operation in

the first decade (from 10 μV to 100 μV). Likewise, the slope can

be raised to 50 mV/dB, using R7 = 3 kΩ, R8 = 2 kΩ , or to

100 mV/dB, to simplify decibel measurements on a DVM,

using R7 = 8 kΩ, R8 = 2 kΩ, which raises the maximum output

11 V, thus requiring a 15 V supply for the AD830. The output

can be made to swing in a negative direction by simply

reversing Pins 1 and 2. Low-pass filtering capacitor, C3, sets the

output rise time to about 1 ms.

6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

1.0

0.5

–0.5

–1.0

10µ

100µ

10m

100m

VIN

Figure 46. Ideal Output and Law-Conformance Error for the DC-Coupled

AD8307 at 50 mV/dB

Figure 46 shows the output and the law-conformance error, in

the absence of noise and input offset, for the 50 mV/dB option.

Note that the error ripple for dc excitation is about twice that

for the more usual sinusoidal excitation. In practice, both the

noise and the internal offset voltage degrade the accuracy in the

first decade of the dynamic range. The latter is now manually

nulled by VR1, using a simple method that ensures very low

residual offsets.

A temporary ac signal, typically a sine wave of 100 mV in

amplitude at a frequency of about 100 Hz, is applied via the

capacitor at node TEMP; this has the effect of disturbing the

offset nulling voltage. The output voltage is then viewed on an

oscilloscope and VR1 is adjusted until the peaks of the

(frequency-doubled) waveform are exactly equal in amplitude.

This procedure can provide an input null down to about 10 μV.

The temperature drift is very low, though not specified since the

AD8307 is not principally designed to operate as a baseband log

amp; in ac modes, this offset is nulled continuously and

automatically.

Next, it is necessary to set the intercept. This is the purpose of

VR2, which should be adjusted after VR1. The simplest method

is to short the input and adjust VR2 for an output of 0.3 V,

corresponding to the noise floor. For more exacting

applications, a temporary sinusoidal test voltage of 1 mV in

amplitude, at about 1 MHz, should be applied, which can

require the use of a temporary on-board input attenuator. For

20 mV/dB scaling, a 10 μV dc intercept (which is 6 dB below

the ac intercept) requires adjusting the output to 0.68 V; for

100 mV/dB scaling, this becomes 3.4 V. If a 100 μV intercept is

preferred (usefully lowering the maximum output voltage),

these become 0.28 V and 1.4 V, respectively.

Finally, the slope must be adjusted. This can be performed by

applying a low frequency square wave to the main input, having

precisely determined upper and lower voltage levels, provided

by a programmable waveform generator. A suitable choice is a

100 Hz square wave with levels of 10 mV and 1 V. The output is

a low-pass filtered square wave, and its amplitude should be

0.8 V for 20 mV/dB scaling, or 4 V for 100 mV/dB scaling.

OPERATION ABOVE 500 MHZ

The AD8307 is not intended for use above 500 MHz. However,

it does provide useful performance at higher frequencies.

Figure 47 shows a plot of the logarithmic output of the AD8307

for an input frequency of 900 MHz. The device shows good

logarithmic conformance from −50 dBm to −10 dBm. There is a

bump in the transfer function at −5 dBm, but if this is

acceptable, the device is usable over a 60 dB dynamic range

(−50 dBm to +10 dBm).

2.0

1.8

1.6

1.4

1.2

0.2

0.4

0.6

0.8

1.0

–60

–50

–40

–30

–20

–10

PIN (dBm)

Figure 47. Output vs. Input Level for a 900 MHz Input Signal

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