Datasheet AD640 (Analog Devices) - 16
| Manufacturer | Analog Devices |
| Description | DC-Coupled Demodulating 120 MHz Logarithmic Amplifier |
| Pages / Page | 19 / 16 — AD640. 10 Hz–100 kHz Converter with 95 dB Dynamic Range. OF FIGURE 32 – … |
| Revision | D |
| File Format / Size | PDF / 421 Kb |
| Document Language | English |
AD640. 10 Hz–100 kHz Converter with 95 dB Dynamic Range. OF FIGURE 32 – V. ERROR FROM IDEAL. TRANSFER FUNCTION – dB
Model Line for this Datasheet
Text Version of Document
AD640
be increased and U3 can be replaced by a low speed op amp. high-pass filter is only operative for very small inputs; see page Figure 31 shows typical performance of this converter. 13.) Figure 33 shows the performance for square wave inputs. Since the attenuator is used, the upper end of the dynamic
10 Hz–100 kHz Converter with 95 dB Dynamic Range
To increase the dynamic range it is necessary to reduce the range now extends to +6 dBV and the intercept is at –82 dBV. bandwidth by the inclusion of a low-pass filter at the signal The noise limited dynamic range is over 100 dB, but in practice interface between U1 and U2 (Figure 32). To provide operation spurious signals at the input will determine the achievable range. down to low frequencies, dc coupling is used at the interface
9 2
between AD640s and the input offset is nulled by a feedback
0
circuit.
8 7 –2
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
6
(AD712) forms a 4-pole 35 Hz low-pass filter. This provides
5
operation to signal frequencies below 20 Hz. The filter response
4
is not critical, allowing the use of an electrolytic capacitor to form one of the poles.
3 OF FIGURE 32 – V
R1 is restricted to 50 Ω by the compliance at Pin 14, so C3
2 ERROR FROM IDEAL TRANSFER FUNCTION – dB
needs to be large to form a 5 ms time constant. A tantalum
LOG OUTPUT FROM CIRCUIT 1
capacitor is used (note polarity). The output of U3a is scaled
0
+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
–1 –90 –80 –70 –60 –50 –40 –30 –20 –10 0 10 dBV
U2 is low-pass filtered by R6/C7 and R7/C8 and buffered by
31.6
m
100
m
316
m
1m 3.16m 10m 31.6m 100m 316m 1 3.16 V INPUT AMPLITUDE AT 10kHz
voltage followers U4a and U4b. The 16s open loop time constant translates to a closed loop high-pass corner of 10 Hz. (This Figure 33. Logarithmic Output and Nonlinearity for Circuit of Figure 32, for a Square Wave Input at f = 10 kHz –16– REV. D Document Outline AD640-SPECIFICATIONS DC Specifications AC Specifications Thermal Characteristics ABSOLUTE MAXIMUM RATINGS TYPICAL DC PERFORMANCE CHARACTERISTICS TYPICAL AC PERFORMANCE CHARACTERISTICS OUTLINE DIMENSIONS ORDERING GUIDE
be increased and U3 can be replaced by a low speed op amp. high-pass filter is only operative for very small inputs; see page Figure 31 shows typical performance of this converter. 13.) Figure 33 shows the performance for square wave inputs. Since the attenuator is used, the upper end of the dynamic
10 Hz–100 kHz Converter with 95 dB Dynamic Range
To increase the dynamic range it is necessary to reduce the range now extends to +6 dBV and the intercept is at –82 dBV. bandwidth by the inclusion of a low-pass filter at the signal The noise limited dynamic range is over 100 dB, but in practice interface between U1 and U2 (Figure 32). To provide operation spurious signals at the input will determine the achievable range. down to low frequencies, dc coupling is used at the interface
9 2
between AD640s and the input offset is nulled by a feedback
0
circuit.
8 7 –2
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
6
(AD712) forms a 4-pole 35 Hz low-pass filter. This provides
5
operation to signal frequencies below 20 Hz. The filter response
4
is not critical, allowing the use of an electrolytic capacitor to form one of the poles.
3 OF FIGURE 32 – V
R1 is restricted to 50 Ω by the compliance at Pin 14, so C3
2 ERROR FROM IDEAL TRANSFER FUNCTION – dB
needs to be large to form a 5 ms time constant. A tantalum
LOG OUTPUT FROM CIRCUIT 1
capacitor is used (note polarity). The output of U3a is scaled
0
+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
–1 –90 –80 –70 –60 –50 –40 –30 –20 –10 0 10 dBV
U2 is low-pass filtered by R6/C7 and R7/C8 and buffered by
31.6
m
100
m
316
m
1m 3.16m 10m 31.6m 100m 316m 1 3.16 V INPUT AMPLITUDE AT 10kHz
voltage followers U4a and U4b. The 16s open loop time constant translates to a closed loop high-pass corner of 10 Hz. (This Figure 33. Logarithmic Output and Nonlinearity for Circuit of Figure 32, for a Square Wave Input at f = 10 kHz –16– REV. D Document Outline AD640-SPECIFICATIONS DC Specifications AC Specifications Thermal Characteristics ABSOLUTE MAXIMUM RATINGS TYPICAL DC PERFORMANCE CHARACTERISTICS TYPICAL AC PERFORMANCE CHARACTERISTICS OUTLINE DIMENSIONS ORDERING GUIDE
