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AD636KH Datasheet(PDF) 6 Page - Analog Devices |
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AD636KH Datasheet(HTML) 6 Page - Analog Devices |
6 / 8 page AD636 REV. B –6– Addition of an external resistor in parallel with RE alters this voltage divider such that increased negative swing is possible. Figure 11 shows the value of REXTERNAL for a particular ratio of VPEAK to –VS for several values of RLOAD. Addition, of REXTERNAL increases the quiescent current of the buffer amplifier by an amount equal to REXT/–VS. Nominal buffer quiescent current with no REXTERNAL is 30 µA at –V S = –5 V. REXTERNAL – 1.0 0.5 0 01M 1k 10k 100k RL = 6.7k RL = 16.7k RL = 50k Figure 11. Ratio of Peak Negative Swing to –VS vs. R EXTERNAL for Several/Load Resistances FREQUENCY RESPONSE The AD636 utilizes a logarithmic circuit in performing the implicit rms computation. As with any log circuit, bandwidth is proportional to signal level. The solid lines in the graph below represent the frequency response of the AD636 at input levels from 1 millivolt to 1 volt rms. The dashed lines indicate the upper frequency limits for 1%, 10%, and ±3 dB of reading additional error. For example, note that a 1 volt rms signal will produce less than 1% of reading additional error up to 220 kHz. A 10 millivolt signal can be measured with 1% of reading addi- tional error (100 µV) up to 14 kHz. FREQUENCY – Hz 1 200m 100m 10m 1m 30m 1k 10k 100k 1M 100 1 VOLT rms INPUT 200mV rms INPUT 100mV rms INPUT 30mV rms INPUT 10mV rms INPUT 1mV rms INPUT 10% 3dB 10M 1% Figure 12. AD636 Frequency Response AC MEASUREMENT ACCURACY AND CREST FACTOR Crest factor is often overlooked in determining the accuracy of an ac measurement. Crest factor is defined as the ratio of the peak signal amplitude to the rms value of the signal (C.F. = VP/ V rms) Most common waveforms, such as sine and triangle waves, have relatively low crest factors (<2). Waveforms that The current mirror also produces the output current, IOUT, which equals 2I4. IOUT can be used directly or converted to a voltage with R2 and buffered by A4 to provide a low impedance voltage output. The transfer function of the AD636 thus results: VOUT = 2 R2 I rms = VIN rms The dB output is derived from the emitter of Q3, since the volt- age at this point is proportional to –log VIN. Emitter follower, Q5, buffers and level shifts this voltage, so that the dB output voltage is zero when the externally supplied emitter current (IREF) to Q5 approximates I3. A4 6 7 5 3 9 8 4 10 14 A1 A2 A3 1 +VS COM RL dB OUT BUF OUT BUFFER BUF IN 10k Q5 Q4 Q2 Q1 Q3 ONE-QUADRANT SQUARER/ DIVIDER CAV IOUT ABSOLUTE VALUE/ VOLTAGE –CURRENT CONVERTER VIN R3 10k 8k 8k + R4 20k |VIN| R4 I1 10 A FS I3 20 A FS R1 25k I4 R2 10k IREF –VS CURRENT MIRROR Figure 9. Simplified Schematic THE AD636 BUFFER AMPLIFIER The buffer amplifier included in the AD636 offers the user additional application flexibility. It is important to understand some of the characteristics of this amplifier to obtain optimum performance. Figure 10 shows a simplified schematic of the buffer. Since the output of an rms-to-dc converter is always positive, it is not necessary to use a traditional complementary Class AB output stage. In the AD636 buffer, a Class A emitter follower is used instead. In addition to excellent positive output voltage swing, this configuration allows the output to swing fully down to ground in single-supply applications without the problems associated with most IC operational amplifiers. CURRENT MIRROR BUFFER INPUT BUFFER OUTPUT +VS RE 40k 10k REXTERNAL (OPTIONAL, SEE TEXT) –VS RLOAD 5 A 5 A Figure 10. AD636 Buffer Amplifier Simplified Schematic When this amplifier is used in dual-supply applications as an input buffer amplifier driving a load resistance referred to ground, steps must be taken to insure an adequate negative voltage swing. For negative outputs, current will flow from the load resistor through the 40 k Ω emitter resistor, setting up a voltage divider between –VS and ground. This reduced effective –VS, will limit the available negative output swing of the buffer. |
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