AD640 datasheet(16/19 Pages) AD

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AD640

REV.

–16–

be increased and U3 can be replaced by a low speed op amp.

Figure 31 shows typical performance of this converter.

10 Hz–100 kHz Converter with 95 dB Dynamic Range

To increase the dynamic range it is necessary to reduce the

bandwidth by the inclusion of a low-pass filter at the signal

interface between U1 and U2 (Figure 32). To provide operation

down to low frequencies, dc coupling is used at the interface

between AD640s and the input offset is nulled by a feedback

circuit.

Using values of 0.02

µF in the interstage filter formed by capaci-

tors C1 and C2, the hf corner occurs at about 100 kHz. U3

(AD712) forms a 4-pole 35 Hz low-pass filter. This provides

operation to signal frequencies below 20 Hz. The filter response

is not critical, allowing the use of an electrolytic capacitor to

form one of the poles.

R1 is restricted to 50

Ω by the compliance at Pin 14, so C3

needs to be large to form a 5 ms time constant. A tantalum

capacitor is used (note polarity). The output of U3a is scaled

+1 V per decade, and the X2 gain of U3b raises this to +2 V per

decade, or +100 mV/dB. The differential offset at the output of

U2 is low-pass filtered by R6/C7 and R7/C8 and buffered by

voltage followers U4a and U4b. The 16s open loop time constant

translates to a closed loop high-pass corner of 10 Hz. (This

high-pass filter is only operative for very small inputs; see page

13.) Figure 33 shows the performance for square wave inputs.

Since the attenuator is used, the upper end of the dynamic

range now extends to +6 dBV and the intercept is at –82 dBV.

The noise limited dynamic range is over 100 dB, but in practice

spurious signals at the input will determine the achievable range.

INPUT AMPLITUDE AT 10kHz

–1

–90

31.6

–80

100

–70

316

–60

–50

3.16m

–40

10m

–30

31.6m

–20

100m

–10

316m

3.16

dBV

–2

Figure 33. Logarithmic Output and Nonlinearity for Circuit

of Figure 32, for a Square Wave Input at f = 10 kHz

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