FEATURES FUNCTIONAL BLOCK DIAGRAM True rms-to-dc conversion 200 mV full scale Laser-trimmed to high accuracy 0.5% maximum error (AD636K) 1.0% maximum error (AD636J) Wide response capability Computes rms of ac and dc signals 1 MHz, −3 dB bandwidth: V rms > 100 mV Signal crest factor of 6 for 0.5% error dB output with 50 dB range Low power: 800 μA quiescent current Single or dual supply operation Monolithic integrated circuit Low cost CAV +VS VIN ABSOLUTE VALUE COM SQUARER DIVIDER +VS CURRENT MIRROR 10kΩ +VS RL IOUT BUFFER IN BUF 10kΩ BUFFER OUT 40kΩ AD636 GENERAL DESCRIPTION –VS The AD636 is a low power monolithic IC that performs true rms-to-dc conversion on low level signals. It offers performance that is comparable or superior to that of hybrid and modular converters costing much more. The AD636 is specified for a signal range of 0 mV to 200 mV rms. Crest factors up to 6 can be accommodated with less than 0.5% additional error, allowing accurate measurement of complex input waveforms. The low power supply current requirement of the AD636, typically 800 μA, is ideal for battery-powered portable instruments. It operates from a wide range of dual and single power supplies, from ±2.5 V to ±16.5 V or from +5 V to +24 V. The input and output terminals are fully protected; the input signal can exceed the power supply with no damage to the device (allowing the presence of input signals in the absence of supply voltage), and the output buffer amplifier is short-circuit protected. The AD636 includes an auxiliary dB output derived from an internal circuit point that represents the logarithm of the rms output. The 0 dB reference level is set by an externally supplied current and can be selected to correspond to any input level from 0 dBm (774.6 mV) to −20 dBm (77.46 mV). Frequency response ranges from 1.2 MHz at 0 dBm to greater than 10 kHz at −50 dBm. The AD636 is easy to use. The device is factory-trimmed at the wafer level for input and output offset, positive and negative waveform symmetry (dc reversal error), and full-scale accuracy at 200 mV rms. Therefore, no external trims are required to achieve full-rated accuracy. Rev. E dB –VS 00787-001 Data Sheet Low Level, True RMS-to-DC Converter AD636 Figure 1. The AD636 is available in two accuracy grades. The total error of the J-version is typically less than ±0.5 mV ± 1.0% of reading, while the total error of the AD636K is less than ±0.2 mV to ±0.5% of reading. Both versions are temperature rated for operation between 0°C and 70°C and available in 14-lead SBDIP and 10-lead TO-100 metal can. The AD636 computes the true root-mean-square of a complex ac (or ac plus dc) input signal and gives an equivalent dc output level. The true rms value of a waveform is a more useful quantity than the average rectified value because it is a measure of the power in the signal. The rms value of an ac-coupled signal is also its standard deviation. The 200 mV full-scale range of the AD636 is compatible with many popular display-oriented ADCs. The low power supply current requirement permits use in battery-powered hand-held instruments. An averaging capacitor is the only external component required to perform measurements to the fully specified accuracy is. Its value optimizes the trade-off between low frequency accuracy, ripple, and settling time. An optional on-chip amplifier acts as a buffer for the input or the output signals. Used in the input, it provides accurate performance from standard 10 MΩ input attenuators. As an output buffer, it sources up to 5 mA. Document Feedback Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 ©2013 Analog Devices, Inc. All rights reserved. Technical Support www.analog.com AD636 Data Sheet TABLE OF CONTENTS Features .............................................................................................. 1 Applications..................................................................................... 10 Functional Block Diagram .............................................................. 1 Standard Connection ................................................................. 10 General Description ......................................................................... 1 Optional Trims for High Accuracy .......................................... 10 Revision History ............................................................................... 2 Single-Supply Connection ........................................................ 10 Specifications..................................................................................... 3 Choosing the Averaging Time Constant ................................. 11 Absolute Maximum Ratings ............................................................ 5 A Complete AC Digital Voltmeter ........................................... 12 ESD Caution .................................................................................. 5 A Low Power, High Input, Impedance dB Meter....................... 12 Pin Configurations and Function Descriptions ........................... 6 Circuit Description ................................................................ 12 Typical Performance Characteristics ............................................. 7 Performance Data .................................................................. 12 Theory of Operation ........................................................................ 8 Frequency Response ±3 dBm ............................................... 13 RMS Measurements ..................................................................... 8 Calibration .............................................................................. 13 The AD636 Buffer Amplifier ...................................................... 8 Outline Dimensions ....................................................................... 14 Frequency Response ..................................................................... 9 Ordering Guide .......................................................................... 14 AC Measurement Accuracy and Crest Factor (CF) ................. 9 REVISION HISTORY 5/13—Rev. D to Rev. E 11/06—Rev. C to Rev. D Reorganized Layout ............................................................ Universal Changes to Figure 1 ........................................................................... 1 Change to Table 1 .............................................................................. 4 Added Typical Performance Characteristics Section ................... 7 Added Theory of Operation Section; Changes to Figure 7 and Figure 8 ............................................................................................... 8 Changed Applying the AD636 Section to Applications Section; Changes to Figure 9, Figure 10, and Single-Supply Connection Section ...............................................................................................10 Changes to Figure 11 .......................................................................11 Changes to Figure 13 and A Complete AC Digital Voltmeter Section ...............................................................................................12 Changes to Figure 17 and Figure 18..............................................13 Changes to Ordering Guide ...........................................................14 Changes to General Description ..................................................... 1 Changes to Table 1............................................................................. 3 Changes to Ordering Guide .......................................................... 13 1/06—Rev B to Rev. C Updated Format .................................................................. Universal Changes to Figure 1 and General Description ..............................1 Deleted Metallization Photograph ..................................................3 Added Pin Configuration and Function Description Section ....6 Updated Outline Dimensions ....................................................... 14 Changes to Ordering Guide .......................................................... 14 8/99—Rev A to Rev. B Rev. E | Page 2 of 16 Data Sheet AD636 SPECIFICATIONS @ 25°C, +VS = +3 V, and −VS = –5 V, unless otherwise noted. 1 Table 1. Model TRANSFER FUNCTION Min AD636J Typ Max Min VOUT = avg × (VIN ) vs. Temperature, 0°C to +70°C ±0.5 ± 1.0 ±0.2 ± 0.5 ±0.1 ± 0.01 ±0.1 ± 0.005 ±0.1 ± 0.01 ±0.1 ± 0.01 DC Reversal Error at 200 mV Total Error, External Trim2 ±0.2 ±0.3 ± 0.3 ±0.1 ± 0.1 ± 0.2 Specified Accuracy −0.2 −0.5 25 Specified Accuracy −0.2 −0.5 25 0 to 200 0 to 200 ±2.8 ±2.0 ±5.0 5.33 6.67 ±12 8 ±0.5 5.33 6.67 mV ± % of reading mV ± % of reading/°C mV ± % of reading/V % of reading mV ± % of reading % of reading % of reading ms/μF of CAV mV rms ±2.8 ±2.0 ±5.0 V p-p V p-p V p-p ±12 8 ±0.2 V p-p kΩ mV 14 90 130 14 90 130 kHz kHz kHz 100 900 1.5 100 900 1.5 kHz kHz MHz ±0.5 ±0.2 ±10 ±0.1 0.3 0.3 8 Unit 2 vs. Supply Voltage ERROR VS. CREST FACTOR 4 Crest Factor 1 to 2 Crest Factor = 3 Crest Factor = 6 AVERAGING TIME CONSTANT INPUT CHARACTERISTICS Signal Range, All Supplies Continuous RMS Level Peak Transient Inputs +3 V, −5 V Supply ±2.5 V Supply ±5 V Supply Maximum Continuous Nondestructive Input Level (All Supply Voltages) Input Resistance Input Offset Voltage FREQUENCY RESPONSE3, 5 Bandwidth for 1% Additional Error (0.09 dB) VIN = 10 mV VIN = 100 mV VIN = 200 mV ±3 dB Bandwidth VIN = 10 mV VIN = 100 mV VIN = 200 mV OUTPUT CHARACTERISTICS3 Offset Voltage, VIN = COM vs. Temperature vs. Supply Voltage Swing +3 V, −5 V Supply ±5 V to ±16.5 V Supply Output Impedance Max VOUT = avg × (VIN ) 2 CONVERSION ACCURACY Total Error, Internal Trim 2, 3 AD636K Typ 0 to 1.0 0 to 1.0 10 12 Rev. E | Page 3 of 16 0.3 0.3 8 ±10 ±0.1 mV μV/°C mV/V 0 to 1.0 0 to 1.0 10 V V kΩ 12 AD636 Data Sheet Model dB OUTPUT Error, VIN = 7 mV to 300 mV rms Scale Factor Scale Factor Temperature Coefficient Min IREF for 0 dB = 0.1 V rms IREF Range IOUT TERMINAL IOUT Scale Factor IOUT Scale Factor Tolerance Output Resistance Voltage Compliance 2 1 BUFFER AMPLIFIER Input and Output Voltage Range Input Offset Voltage, RS = 10 kΩ Input Bias Current Input Resistance Output Current Short-Circuit Current Small Signal Bandwidth Slew Rate 6 POWER SUPPLY Voltage, Rated Performance Dual Supply Single Supply Quiescent Current 7 TEMPERATURE RANGE Rated Performance Storage TRANSISTOR COUNT AD636J Typ Min ±0.5 ±0.3 −3.0 0.33 −20 8 Max −0.033 4 100 ±10 10 −VS to (+VS − 2 V) AD636K Typ ±0.1 −3.0 0.33 8 50 2 1 +20 12 −20 8 −0.033 4 100 ±10 10 −VS to (+VS − 2 V) Max Unit ±0.2 dB mV/dB % of reading/°C 8 50 +20 12 −VS to (+VS − 2 V) −VS to (+VS − 2 V) ±0.8 100 108 ±2 300 (+5 mA, −130 μA) dB/°C μA μA μA/V rms % kΩ V V ±0.5 100 108 ±1 300 mV nA Ω (+5 mA, −130 μA) 20 1 5 20 1 5 +3, −5 +2, −2.5 5 0.80 0 −55 mA MHz V/μs +3, −5 ±16.5 24 1.00 +2, −2.5 5 +70 +150 0 −55 62 0.80 ±16.5 24 1.00 V V V mA +70 +150 °C °C 62 All minimum and maximum specifications are guaranteed. Specifications shown in boldface are tested on all production units at final electrical test and are used to calculate outgoing quality levels. Accuracy specified for 0 mV to 200 mV rms, dc or 1 kHz sine wave input. Accuracy is degraded at higher rms signal levels. 3 Measured at Pin 8 of PDIP (IOUT), with Pin 9 tied to common. 4 Error vs. crest factor is specified as additional error for a 200 mV rms rectangular pulse train, pulse width = 200 µs. 5 Input voltages are expressed in V rms. 6 With 10 kΩ pull-down resistor from Pin 6 (BUF OUT) to −VS. 7 With BUF IN tied to COMMON. 1 2 Rev. E | Page 4 of 16 Data Sheet AD636 ABSOLUTE MAXIMUM RATINGS Table 2. Parameter Supply Voltage Dual Supply Single Supply Internal Power Dissipation 1 Maximum Input Voltage Storage Temperature Range Operating Temperature Range Lead Temperature Range (Soldering 60 sec) ESD Rating 1 Ratings ±16.5 V 24 V 500 mW ±12 VPEAK −55°C to +150°C 0°C to 70°C 300°C 1000 V Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ESD CAUTION 10-Lead TO: θJA = 150°C/W. 14-Lead PDIP: θJA = 95°C/W. Rev. E | Page 5 of 16 AD636 Data Sheet PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS 14 +VS NC 2 13 NC 12 NC –VS 3 AD636 RL TOP VIEW 11 NC (Not to Scale) dB 5 10 COM RL BUF IN 7 8 IOUT NC = NO CONNECT COM BUF OUT 8 7 1 AD636 6 2 3 +VS 00787-003 9 9 10 CAV 4 BUF OUT 6 BUF IN IOUT dB CAV 5 4 VIN –VS 00787-004 VIN 1 Figure 3. 10-Pin TO-100 Pin Configuration Figure 2. 14-Lead SBDIP Pin Configuration Table 3. Pin Function Descriptions—14-Lead SBDIP Table 4. Pin Function Descriptions—10-Pin TO-100 Pin No. 1 2 3 4 5 Mnemonic VIN NC −VS CAV dB 6 7 8 9 10 11, 12, 13 14 BUF OUT BUF IN IOUT RL COM NC +VS Pin No. 1 2 3 4 5 6 7 8 9 10 Description Input Voltage. No Connection. Negative Supply Voltage. Averaging Capacitor. Log (dB) Value of the RMS Output Voltage. Buffer Output. Buffer Input. RMS Output Current. Load Resistor. Common. No Connection. Positive Supply Voltage. Rev. E | Page 6 of 16 Mnemonic RL COM +VS VIN −VS CAV dB BUF OUT BUF IN IOUT Description Load Resistor. Common. Positive Supply Voltage. Input Voltage. Negative Supply Voltage. Averaging Capacitor. Log (dB) Value of the RMS Output Voltage. Buffer Output. Buffer Input. RMS Output Current. Data Sheet AD636 TYPICAL PERFORMANCE CHARACTERISTICS 0.5 0.5 RL = 16.7kΩ 0 1k 10k REXTERNAL (Ω) 100k 1M Figure 4. Ratio of Peak Negative Swing to −VS vs. REXTERNAL for Several Load Resistances 200m 1% 10% ±3dB 200mV rms INPUT 100mV rms INPUT 100m 30mV rms INPUT 30m 10m 10mV rms INPUT 1m 0.1m 1k 10k 100k FREQUENCY (Hz) 1M 10M 00787-016 1mV rms INPUT Figure 5. AD636 Frequency Response Rev. E | Page 7 of 16 T EIN (rms) = 200mV 200µs EO –0.5 1 2 3 4 5 CREST FACTOR Figure 6. Error vs. Crest Factor 1V rms INPUT 1 0 –1.0 00787-015 0 0 CF = 1/ ŋ VP 200µs 6 7 00787-017 RL = 50kΩ RL = 6.7kΩ VOUT (V) ŋ = DUTY CYCLE = T INCREASE IN ERROR (% of Reading) RATIO OF VPEAK /VSUPPLY 1.0 AD636 Data Sheet THEORY OF OPERATION CURRENT MIRROR The AD636 embodies an implicit solution of the rms equation that overcomes the dynamic range as well as other limitations inherent in a straightforward computation of rms. The actual computation performed by the AD636 follows the equation: I3 V 2 V rms Avg IN V rms ABSOLUTE VALUE/ VOLTAGE–CURRENT CONVERTER The AD636 is comprised of four major sections: absolute value circuit (active rectifier), squarer/divider, current mirror, and buffer amplifier (see Figure 7, for a simplified schematic). The input voltage, VIN, which can be ac or dc, is converted to a unipolar current I1, by the active rectifier A1, A2. I1 drives one input of the squarer/divider, which has the transfer function: A1 8 R2 10kΩ CAV CAV IOUT IREF A3 Q3 BUF IN BUFFER 7 R4 8kΩ Q2 Q4 8kΩ COM A4 9 RL +VS 5 dB OUT 6 BUF OUT 3 –VS Q5 10kΩ A2 R3 10kΩ 10 ONE-QUADRANT SQUARER/ DIVIDER Figure 7. Simplified Schematic I12 I3 THE AD636 BUFFER AMPLIFIER The output current, I4, of the squarer/divider drives the current mirror through a low-pass filter formed by R1 and the externally connected capacitor, CAV. If the R1, CAV time constant is much greater than the longest period of the input signal, then I4 is effectively averaged. The current mirror returns a current I3, which equals Avg. [I4], back to the squarer/divider to complete the implicit rms computation. Therefore, I22 I4 Avg I1 rms I4 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 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 8 shows a simplified schematic of the buffer. Because 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. +VS CURRENT MIRROR VOUT = 2 R2 I rms = VIN rms 5µA The dB output is derived from the emitter of Q3, because the voltage 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. 5µA BUFFER INPUT 10kΩ RE 40kΩ –VS BUFFER OUTPUT RLOAD REXTERNA L (OPTIONAL, SEE TEXT) 00787-014 I4 VIN 1 I1 |VIN| + 4 I4 Q1 R4 20kΩ +VS 20µA FS R1 25kΩ 10µA FS 14 Figure 8. 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 ensure an adequate negative voltage swing. For negative outputs, current flows from the load resistor through the 40 kΩ emitter resistor, setting up a voltage divider between −VS and ground. This reduced effective −VS, limits the available negative output swing of the buffer. The addition of an external resistor in parallel with RE alters this voltage divider such that increased negative swing is possible. Rev. E | Page 8 of 16 00787-013 RMS MEASUREMENTS Data Sheet AD636 Figure 4 shows the value of REXTERNAL for a particular ratio of VPEAK to −VS for several values of RLOAD. The 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 −VS = −5 V. FREQUENCY RESPONSE The AD636 uses a logarithmic circuit to perform the implicit rms computation. As with any log circuit, bandwidth is proportional to signal level. The solid lines in Figure 5 represent the frequency response of the AD636 at input levels from 1 mV to 1 V 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 V rms signal produces less than 1% of reading additional error up to 220 kHz. A 10 mV signal can be measured with 1% of reading additional error (100 µV) up to 14 kHz. AC MEASUREMENT ACCURACY AND CREST FACTOR (CF) 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 (CF = VP/V rms). Most common waveforms, such as sine and triangle waves, have relatively low crest factors (<2). Waveforms that resemble low duty cycle pulse trains, such as those occurring in switching power supplies and SCR circuits, have high crest factors. For example, a rectangular pulse train with a 1% duty cycle has a crest factor of 10 (CF = 1/√η). Figure 6 is a curve of reading error for the AD636 for a 200 mV rms input signal with crest factors from 1 to 7. A rectangular pulse train (pulse width 200 μs) was used for this test because it is the worst-case waveform for rms measurement (all the energy is contained in the peaks). The duty cycle and peak amplitude were varied to produce crest factors from 1 to 7 while maintaining a constant 200 mV rms input amplitude. Rev. E | Page 9 of 16 AD636 Data Sheet APPLICATIONS The trimming procedure is as follows: • Ground the input signal, VIN, and adjust R4 to give 0 V output from Pin 6. Alternatively, R4 can be adjusted to give the correct output with the lowest expected value of VIN. • Connect the desired full-scale input level to VIN, either dc or a calibrated ac signal (1 kHz is the optimum frequency); then trim R1 to give the correct output from Pin 6, that is, 200 mV dc input should give 200 mV dc output. Of course, a ±200 mV peak-to-peak sine wave should give a 141.4 mV dc output. The remaining errors, as given in the specifications, are due to the nonlinearity. STANDARD CONNECTION The AD636 is simple to connect for the majority of high accuracy rms measurements, requiring only an external capacitor to set the averaging time constant. The standard connection is shown in Figure 9 In this configuration, the AD636 measures the rms of the ac and dc level present at the input but shows an error for low frequency inputs as a function of the filter capacitor, CAV, as shown in Figure 13. Therefore, if a 4 μF capacitor is used, the additional average error at 10 Hz is 0.1%, and at 3 Hz it is 1%. The accuracy at higher frequencies is according to specification. If it is desired to reject the dc input, a capacitor is added in series with the input, as shown in Figure 11; the capacitor must be nonpolar. If the AD636 is driven with power supplies with a considerable amount of high frequency ripple, it is advisable to bypass both supplies to ground with 0.1 μF ceramic discs as near the device as possible. CF is an optional output ripple filter. CF (OPTIONAL) +V 3 C SQUARER DIVIDER dB 5 BUF OUT BUF IN CURRENT MIRROR 6 7 10 9 + BUF – 10kΩ 2 10kΩ 8 +V COM RL CF (OPTIONAL) NC = NO CONNECT 3 erms ABSOLUTE VALUE –VS + –V – NC 2 –VS 3 CAV dB 5 SQUARER DIVIDER 12 NC 11 NC CURRENT MIRROR 10 BUF OUT VOUT BUF IN 6 7 +V 14 13 NC AD636 4 9 + BUF – 10kΩ 10kΩ 8 COM R2 RL 154Ω IOUT R3 470kΩ +VS R4 500kΩ –VS OFFSET ADJUST Figure 10. Optional External Gain and Output Offset Trims BUF IN BUF OUT 8 5 VOUT 10kΩ SQUARER DIVIDER VIN 4 IOUT 9 + – BUF CURRENT MIRROR +VS +VS ABSOLUTE VALUE NC = NO CONNECT 10kΩ AD636 COM 12 NC 11 NC 4 – 1 VIN 1 R1 200Ω ±1.5% 10 +V 14 13 NC NC 2 AD636 –VS CAV + ABSOLUTE VALUE CAV – + erms IOUT 7 dB 6 CAV –V 00787-005 1 –V RL +VS VIN erms SCALE FACTOR ADJUST 00787-006 The input and output signal ranges are a function of the supply voltages as detailed in the specifications. The AD636 can also be used in an unbuffered voltage output mode by disconnecting the input to the buffer. The output then appears unbuffered across the 10 kΩ resistor. The buffer amplifier can then be used for other purposes. Further, the AD636 can be used in a current output mode by disconnecting the 10 kΩ resistor from the ground. The output current is available at Pin 8 (Pin 10 on the H package) with a nominal scale of 100 μA per volt rms input, positive out. CAV Figure 9. Standard RMS Connection OPTIONAL TRIMS FOR HIGH ACCURACY If it is desired to improve the accuracy of the AD636, the external trims shown in Figure 10 can be added. R4 is used to trim the offset. The scale factor is trimmed by using R1 as shown. The insertion of R2 allows R1 to either increase or decrease the scale factor by ±1.5%. SINGLE-SUPPLY CONNECTION Although the applications illustrated in Figure 9 and Figure 10 assume the use of dual power supplies, three external bias components connected to the COM pin enable powering the AD636 with unipolar supplies as low as 5 V. The two resistors and capacitor network shown connected to Pin 10 in Figure 11 are satisfactory over the same range of voltages permissible with dual supply operation. Any external bias voltage applied to Pin 10 is internally reflected to the VIN pin, rendering the same ac operation as with a dual supply. DC or ac + dc conversion is impractical, due to the resultant dc level shift at the input. The capacitor insures that no extraneous signals are coupled into the COM pin. The values of the resistors are relatively high to minimize power consumption because only 1 µA of bias current flows into Pin 10 (Pin 2 on the H package). Alternately, the COM pin of some CMOS ADCs provides a suitable artificial ground for the AD636. AC input coupling requires only Capacitor C2 as shown; a dc return is not necessary because it is provided internally. C2 is selected for the proper low frequency break point with the input resistance of 6.7 kΩ; for a cut-off at 10 Hz, C2 should be 3.3 μF. The signal ranges in this connection are Rev. E | Page 10 of 16 Data Sheet AD636 % 01 0. 8 IOUT REQUIRED CAV (µF) Figure 11. Single-Supply Connection (See Text) CHOOSING THE AVERAGING TIME CONSTANT The AD636 computes the rms of both ac and dc signals. If the input is a slowly varying dc voltage, the output of the AD636 tracks the input exactly. At higher frequencies, the average output of the AD636 approaches the rms value of the input signal. The actual output of the AD636 differs from the ideal output by a dc (or average) error and some amount of ripple, as demonstrated in Figure 12. EO DC ERROR = EO – EO (IDEAL) DOUBLE-FREQUENCY RIPPLE AVERAGE EO = EO TIME 00787-008 IDEAL EO 100 1k INPUT FREQUENCY (Hz) 10k 0.01 100k Figure 13. Error/Settling Time Graph for Use with the Standard RMS Connection 39kΩ NC = NO CONNECT 10 0.1 The primary disadvantage in using a large CAV to remove ripple is that the settling time for a step change in input level is increased proportionately. Figure 13 shows the relationship between CAV and 1% settling time is 115 ms for each microfarad of CAV. The settling time is twice as great for decreasing signals as for increasing signals (the values in Figure 13 are for decreasing signals). Settling time also increases for low signal levels, as shown in Figure 14. Figure 12. Typical Output Waveform for Sinusoidal Input 10.0 7.5 5.0 2.5 1.0 The dc error is dependent on the input signal frequency and the value of CAV. Figure 13 can be used to determine the minimum value of CAV, which yields a given % dc error above a given frequency using the standard rms connection. The ac component of the output signal is the ripple. There are two ways to reduce the ripple. The first method involves using a large value of CAV. Because the ripple is inversely proportional to CAV, a tenfold increase in this capacitance effects a tenfold reduction in ripple. When measuring waveforms with high crest factors (such as low duty cycle pulse trains), the averaging time constant should be at least ten times the signal period. For example, a 100 Hz pulse rate requires a 100 ms time constant, which corresponds to a 4 μF capacitor (time constant = 25 ms per μF). 0 1mV 10mV 100mV rms INPUT LEVEL 1V 00787-010 10kΩ 1 SETTLING TIME RELATIVE TO SETTLING TIME @ 200mV rms + BUF – 10kΩ 0.01 0.1µF 00787-007 7 COM RL R BUF IN 9 6 R RL 1kΩ TO 10kΩ 10 R CURRENT MIRROR O R ER BUF OUT 20kΩ 1 VALUES FOR CAV AND 1% SETTLING TIME FOR STATED % OF READING AVERAGING ERROR* ACCURACY ±20% DUE TO COMPONENT TOLERANCE *% dc ERROR + % RIPPLE (PEAK) 0.1 11 NC 4 dB 5 VOUT 12 NC 1 R SQUARER DIVIDER O R ER CAV 13 NC AD636 3 0.1µF 10 O R ER NC 2 14 % 10 ABSOLUTE VALUE 1% 0. +VS 1 NONPOLARIZED –VS 10 1% VIN VIN O R ER CAV – + C2 3.3µF 100 00787-009 100 FOR 1% SETTLING TIME IN SECONDS MULTIPLY READING BY 0.115 slightly more restricted than in the dual supply connection. The load resistor, RL, is necessary to provide current sinking capability. Figure 14. Settling Time vs. Input Level A better method for reducing output ripple is the use of a postfilter. Figure 15 shows a suggested circuit. If a single-pole filter is used (C3 removed, RX shorted), and C2 is approximately 5 times the value of CAV, the ripple is reduced, as shown in Figure 16, and the settling time is increased. For example, with CAV = 1 µF and C2 = 4.7 μF, the ripple for a 60 Hz input is reduced from 10% of reading to approximately 0.3% of reading. The settling time, however, is increased by approximately a factor of 3. The values of CAV and C2 can therefore be reduced to permit faster settling times while still providing substantial ripple reduction. Rev. E | Page 11 of 16 AD636 Data Sheet Calibration of the dB range is accomplished by adjusting R9 for the desired 0 dB reference point, and then adjusting R14 for the desired dB scale factor (a scale of 10 counts per dB is convenient). The 2-pole post filter uses an active filter stage to provide even greater ripple reduction without substantially increasing the settling times over a circuit with a 1-pole filter. The values of CAV, C2, and C3 can then be reduced to allow extremely fast settling times for a constant amount of ripple. Caution should be exercised in choosing the value of CAV, because the dc error is dependent upon this value and is independent of the post filter. For a more detailed explanation of these topics, refer to the RMS-to-DC Conversion Application Guide, 2nd Edition. VIN NC +VS – CAV + 2 3 dB 5 7 + – AD636 SQUARER DIVIDER CURRENT MIRROR 13 NC 12 NC 11 NC In this circuit, the built-in buffer amplifier of the AD636 is used as a bootstrapped input stage increasing the normal 6.7 kΩ input Z to an input impedance of approximately 1010 Ω. COM 10 10kΩ + BUF – 10kΩ +V 14 9 6 BUF IN C2 ABSOLUTE VALUE 4 C BUF OUT The portable dB meter circuit combines the functions of the AD636 rms converter, the AD589 voltage reference, and a μ A77 6 low power operational amplifier (see Figure 18). This meter offers excellent bandwidth and superior high and low level accuracy while consuming minimal power from a standard 9 V transistor radio battery. +VS 1 –VS –V A LOW POWER, HIGH INPUT, IMPEDANCE dB METER 8 Rx 10kΩ RL IOUT C3 (FOR SINGLE POLE, SHORT Rx, REMOVE C3) Circuit Description – + 00787-011 VIN Total power supply current for this circuit is typically 2.8 mA using a 7106-type ADC. Vrms OUT NC = NO CONNECT 10 p-p RIPPLE (ONE POLE) CAV = 1µF C2 = 4.7µF DC ERROR CAV = 1µF (ALL FILTERS) 1 p-p RIPPLE (TWO POLE) CAV = 1µF, C2 = C3 = 4.7µF 0.1 10 100 1k FREQUENCY (Hz) 10k The buffer’s output, Pin 6, is ac-coupled to the rms converter’s input (Pin 1) by capacitor C2. Resistor R9 is connected between the buffer’s output, a Class A output stage, and the negative output swing. Resistor R1 is the amplifier’s bootstrapping resistor. With this circuit, single-supply operation is made possible by setting ground at a point between the positive and negative sides of the battery. This is accomplished by sending 250 μA from the positive battery terminal through R2, then through the 1.2 V AD589 band gap reference, and finally back to the negative side of the battery via R10. This sets ground at 1.2 V + 3.18 V (250 μA × 12.7 kΩ) = 4.4 V below the positive battery terminal and 5.0 V (250 μA × 20 kΩ) above the negative battery terminal. Bypass capacitors, C3 and C5, keep both sides of the battery at a low ac impedance to ground. The AD589 band gap reference establishes the 1.2 V regulated reference voltage, which together with R3 and trimming Potentiometer R4, sets the 0 dB reference current, IREF. p-p RIPPLE CAV = 1µF (STANDARD CONNECTION) 00787-012 DC ERROR OR RIPPLE (% of Reading) Figure 15. 2-Pole Post Filter The input voltage, VIN, is ac-coupled by C4 while R8, together with D1 and D2, provide high input voltage protection. Figure 16. Performance Features of Various Filter Types A COMPLETE AC DIGITAL VOLTMETER Figure 17 shows a design for a complete low power ac digital voltmeter circuit based on the AD636. The 10 MΩ input attenuator allows full-scale ranges of 200 mV, 2 V, 20 V, and 200 V rms. Signals are capacitively coupled to the AD636 buffer amplifier, which is connected in an ac bootstrapped configuration to minimize loading. The buffer then drives the 6.7 kΩ input impedance of the AD636. The COM terminal of the ADC provides the false ground required by the AD636 for singlesupply operation. An AD589 1.2 V reference diode is used to provide a stable 100 mV reference for the ADC in the linear rms mode; in the dB mode, a 1N4148 diode is inserted in series to provide correction for the temperature coefficient of the dB scale factor. Adjust R13 to calibrate the meter for an accurate readout at full scale. Performance Data 0 dB Reference Range = 0 dBm (770 mV) to −20 dBm (77 mV) rms 0 dBm = 1 mW in 600 Ω Input Range (at IREF = 770 mV) = 50 dBm Input Impedance = approximately 1010 VSUPPLY Operating Range = +5 V dc to +20 V dc IQUIESCENT = 1. 8 mA typical Accuracy with 1 kHz sine wave and 9 V dc supply: 0 dB to −40 dBm ± 0.1 dBm 0 dBm to −50 dBm ± 0.15 dBm +10 dBm to −50 dBm ± 0.5 dBm Rev. E | Page 12 of 16 Data Sheet AD636 This can be anywhere from 0 dBm (770 mV rms − 2.2 V p-p) to −20 dBm (77 mV rms − 220 mV p-p). Adjust the IREF calibration trimmer for a zero indication on the analog meter. Frequency Response ±3 dBm Input 0 dBm = 5 Hz to 380 kHz −10 dBm = 5 Hz to 370 kHz −20 dBm = 5 Hz to 240 kHz −30 dBm = 5 Hz to 100 kHz −40 dBm = 5 Hz to 45 kHz −50 dBm = 5 Hz to 17 kHz Then, calibrate the meter scale factor or gain. Apply an input signal −40 dB below the set 0 dB reference and adjust the scale factor calibration trimmer for a 40 μA reading on the analog meter. The temperature compensation resistors for this circuit can be purchased from Micro-Ohm Corporation, 1088 Hamilton Rd., Duarte, CA 91010, Part #Type 401F, 2 kΩ ,1% + 3500 ppm/°C. Calibration First, calibrate the 0 dB reference level by applying a 1 kHz sine wave from an audio oscillator at the desired 0 dB amplitude. D1 1N4148 C4 2.2µF + – R6 1MΩ VIN 1 C3 0.02µF NC 2V –VS R2 900kΩ – 20V CAV 6.8µF + 2 SQUARER DIVIDER 4 dB CURRENT MIRROR 5 R3 90kΩ BUF OUT 200V BUF IN R4 10kΩ 11 NC R10 20kΩ 9 7 R7 20kΩ COM 12 NC R9 100kΩ 0dB SET 10 6 + BUF – 10kΩ R8 2.49kΩ 13 NC AD636 3 +VS +VS 10kΩ 8 R11 10kΩ R12 1kΩ RL R13 500Ω + ON REF HI dB R14 10kΩ dB SCALE 9V BATTERY LIN SCALE LIN dB R15 1MΩ LIN C7 6.8µF 3-1/2 DIGIT LCD DISPLAY HI ANALOG IN C6 0.01µF LO –VS LXD 7543 –VSS Figure 17. Portable, High-Z Input, RMS DPM and dB Meter Circuit R1 1MΩ C2 6.8µF + SIGNAL INPUT + VIN NC 2 –VS CAV 3 ABSOLUTE VALUE 14 13 AD636 R2 12.7kΩ NC 4 12 NC 11 NC C3 10µF + dB R8 47kΩ 1W BUF OUT BUF IN R9 10kΩ 5 CURRENT MIRROR 6 7 10 9 + BUF – 10kΩ R3 5kΩ AD589J *R7 2kΩ COM RL + 10kΩ 8 IOUT – 9V +1.2V + SQUARER DIVIDER ON/OFF + +4.2V +VS 250µA C4 0.1µF D2 1N6263 1 + REF LO dB C1 3.3µF – COM IOUT D4 1N4148 D1 1N6263 OFF +VDD +VDD 1µF 3-1/2 DIGIT 7106 TYPE A/D CONVERTER –VSS LIN D3 1.2V AD589 COM + NC = NO CONNECT D2 1N4148 R6 100Ω C5 10µF C6 0.1µF SCALE FACTOR ADJUST R4 500kΩ IREF ADJUST R5 10kΩ 100µA + 7 – µA776 8 3 + 2 – 0–50µA 6 4 R10 20kΩ R11 820kΩ 5% NC = NO CONNECT –4.8V ALL RESISTORS 1/4W 1% METAL FILM UNLESS OTHERWISE STATED EXCEPT *WHICH IS 2kΩ +3500ppm 1% TC RESISTOR. Figure 18. Low Power, High Input Impedance dB Meter Rev. E | Page 13 of 16 00787-019 R1 9MΩ 14 ABSOLUTE VALUE 00787-018 200mV VIN R5 47kΩ 1W 10% AD636 Data Sheet OUTLINE DIMENSIONS 0.005 (0.13) MIN 0.080 (2.03) MAX 8 14 1 PIN 1 0.200 (5.08) MAX 7 0.310 (7.87) 0.220 (5.59) 0.100 (2.54) BSC 0.765 (19.43) MAX 0.200 (5.08) 0.125 (3.18) 0.023 (0.58) 0.014 (0.36) 0.070 (1.78) 0.030 (0.76) 0.320 (8.13) 0.290 (7.37) 0.060 (1.52) 0.015 (0.38) 0.150 (3.81) MIN SEATING PLANE 0.015 (0.38) 0.008 (0.20) CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN. Figure 19. 14-Lead Side-Brazed Ceramic Dual In-Line Package [SBDIP] (D-14) Dimensions shown in inches and (millimeters) REFERENCE PLANE 0.500 (12.70) MIN 0.185 (4.70) 0.165 (4.19) 0.160 (4.06) 0.110 (2.79) 0.335 (8.51) 0.305 (7.75) 0.370 (9.40) 0.335 (8.51) 6 7 5 0.021 (0.53) 0.016 (0.40) 0.115 (2.92) BSC 8 4 9 0.045 (1.14) 0.025 (0.65) 3 2 0.040 (1.02) MAX BASE & SEATING PLANE 10 1 0.034 (0.86) 0.025 (0.64) 0.230 (5.84) BSC 36° BSC DIMENSIONS PER JEDEC STANDARDS MO-006-AF CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN. 022306-A 0.050 (1.27) MAX Figure 20. 10-Pin Metal Header Package [TO-100] (H-10) Dimensions shown in inches and (millimeters) ORDERING GUIDE Model 1 AD636JDZ AD636KDZ AD636JH AD636JHZ AD636KH AD636KHZ 1 Temperature Range 0°C to +70°C 0°C to +70°C 0°C to +70°C 0°C to +70°C 0°C to +70°C 0°C to +70°C Package Description 14-Lead SBDIP 14-Lead SBDIP 10-Pin TO-100 10-Pin TO-100 10-Pin TO-100 10-Pin TO-100 Z = RoHS-Compliant Part. Rev. E | Page 14 of 16 Package Option D-14 D-14 H-10 H-10 H-10 H-10 Data Sheet AD636 NOTES Rev. E | Page 15 of 16 AD636 Data Sheet NOTES ©2013 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D00787-0-5/13(E) Rev. E | Page 16 of 16