ES636

ES636 True RMS-to-DC Converters Features • Description The ES636 is a true RMS-to-DC converter. It accepts low-level input signals from 0 to 400 mV RMS complex input waveforms. It can be operated form either a single supply or dual supplies. The device draws less than 1 mA of quiescent supply current, furthermore, an enable pin is provided to turn-off the device, making it ideal for battery-powered applications. True RMS-to-DC Conversion • Computes RMS of AC and DC Signals • Wide Response: * 1MHz Bandwidth for VRMS > 100mV • Auxiliary dB Output: * 50dB Range • • • • Single-or Dual-Supply Operation Low Cost Power-Down Function Low Power: 800μA typical Application * Digital Multi-Meters * Battery-Powered Instruments * Panel Meter 1 09/02/16 ES636 True RMS-to-DC Converters Pin Assignment 1 VIN +Vs 14 ES636 2 2 Enable N.C. 13 3 -Vs N.C. 12 4 CAV N.C. 11 5 dB COMMON 10 6 BUF out RL 9 7 BUF in Iout 8 SOP 14 Pin Package 09/02/16 ES636 True RMS-to-DC Converters Pin Description Pin No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Symbol Vin Enable -Vs Cav dB BUF OUT BUF IN Iout RL COMMON N.C. N.C. N.C +Vs Type I I P IO IO O I O IO G Description Measurement input Chip enable, active LOW Negative supply voltage Averaging capacitor DB out Buffer output Buffer input Rms output RL terminal, connected to COMMON in general Analog ground P Positive supply voltage Absolute Maximum Ratings S upply Voltage: Dual Supplies ......…………………………….….... ± 10V Single Supply ..……………………...…….....……+20V Input Voltage: ...................………………………….……………..... ± 10V Power Dissipation (Package) SOP………………………………………………………………...450mW Operating Temperature Range .......……………………………….......0℃ to +70℃ Storage Temperature Range.....…………………...……………............-55℃ to +150℃ Lead Temperature (Soldering, 10sec)....…………………………………...............300℃ 3 09/02/16 ES636 True RMS-to-DC Converters Electrical Characteristics-ES636 (TA= +25℃, Vs = +3V, -Vs = -3V, unless otherwise noted.) PARAMETER Transfer Equation Averaging Time Constant CONVERSION ACCURACY Total Error, Internal Trim (Notes 1,2) Total Error vs. Temperature (0 ℃ to + 70℃) Total Error vs. Supply Total Error vs. DC Reversal Total Error, External Trim (Note 1) VIN=+400mV DC ±0.5 ±1.0 ±0.1 ±0.01 mV ±% of Reading mV ±% of Reading/℃ mV ±% of Reading/V ±2.0 ±0.5 ±0.2 ±% of Reading mV ±% of Reading Figure 3 CONDITIONS MIN TYP 6 MAX UNITS ms/μF CAV VOUT = [avg.(VIN)2]1/2 ±0.1 ±0.01 4 09/02/16 ES636 True RMS-to-DC Converters Electrical Characteristics-ES636(continued) (TA= +25℃, Vs = +3V, -Vs = -3V, unless otherwise noted.) PARAMETER ERROR vs. CREST FACTOR Additional Error CONDITIONS MIN TYP MAX UNITS Crest Factor 1 to 2 Crest Factor = 3 Crest Factor = 6 FREQUENCY RESPONSE (Note 2,4) VIN =35mV Bandwidth for 1% Additional VIN=100mV Error (0.09dB) VIN =400mV ±3dB Bandwidth INPUT CHARACTERISTICS Continuous RMS, All Supplies +3V, -5V Supplies Peak Transient ±2.5V Supplies ±5V Supplies Safe Input Input Resistance Input Offset Voltage All Supplies VIN =35mV VIN =100mV VIN =400mV Specified Accuracy 0.2 0.5 75 99 560 1.1 2.5 6.5 0 to 400 ±2.8 ±2 ±5 ±12 8 ±0.5 ±% of Reading kHz MHz MHz mVRMS VPK VPK kΩ mV Input Signal range 5.33 6.7 OUTPUT CHARACTERISTICS (Note 1) TA=+25℃ Offset Voltage TA =TMIN to TMAX With Supply Voltage +3V, -3V Supplies ±5V to ±10V Supplies ±10 ±0.1 0 to 2 0 to 1 8 1.5 10 ±0.3 -3 ±0.5 mV μV/℃ mV/V V Output Voltage Swing Output Resistance dB OUTPUT ERROR Scale Factor Scale Factor Tempco IREF IREF Range IOUT TERMINAL IOUT Scale Factor IOUT Scale Factor Tolerance Output Resistance Voltage Compliance BUFFER AMPLIFIER Input and Output Voltage Range Input Offset Voltage Input Current Input Resistance Output Current Short-Circuit Current Small-Signal Bandwidth Slew Rate (Note 5) 12 ±0.5 kΩ dB mV/dB %/℃ dB/℃ μA μA μA/VRMS % kΩ V 3mV ≦ VIN ≦ 1V 0dB=0.11VRMS 2 1 4 8 50 -20 8 125 ±10 10 -Vs to (+Vs-2.0) ±20 12 -Vs to (+Vs-2.0) Rs=10kΩ ±0.8 100 8 10 Source Sink +2 -130 20 1 3 ±2 300 V mV nA Ω mA μA mA MHz V/μs 5 09/02/16 ES636 True RMS-to-DC Converters Electrical Characteristics-ES636(continued) (TA= +25℃, Vs = +3V, -Vs = -3V, unless otherwise noted.) Power SUPPLY Rated Performance Dual Supplies Single Supply Quiescent Current (Note 6) Note 2: Measured at pin 8 (IOUT), with pin 9 tied to COMMON. Note 3: Error vs. crest factor is specified as an additional error for 200mVRMS rectangular pulse input, pulse width = 200μs. Note 4: Input voltages are expressed in volts RMS. Note 5: With 10 kΩ external pull-down resistor from pin 6 (BUF OUT) to – Vs. Note 6: With BUF input tied to COMMON. +2/-2.5 +5 0.8 +3/-3 ±10 +20 1 V V V mA Note 1: Accuracy is specified for 0 to 400mV, 1kHz sine-wave input. Accuracy is degraded at higher RMS signal levels. Detailed Description Figure 1 shows the simplified schematic of ES636. It consists of four major subcircuits: absolute value circuit (rectifier), square/divider, current mirror and buffer amplifier. The actual computation performed by the ES636 follows the equation: VRMS = Avg. [VIN2／VRMS] The input voltage, VIN, applied to the ES636 is converted to a unipolar current I1 (Figure 1) by the absolute-value/voltage. This current drives one input of the squarer/divider that produces a current I4 , which has the transfer function: Ι4 = 2 Ι1 Ι3 The current I4 drives the internal current mirror through a low-pass filter formed by R1 and the external capacitor, CAV. As long as the time constant of this filter is greater than the longest period of the input signal, I4 is averaged. The current mirror returns a current, I3, to the square/divider to complete the circuit. The current I4 is then a function of the average of (I12/ I4), which is equal to I1RMS. The current mirror also produces a 2．I4 output current, IOUT, that can be used directly or converted to a voltage using resistor R2 and the internal buffer to provide a low-impedance voltage output. The transfer function for the ES636 is: VOUT=2．R2．IRMS= VIN 6 09/02/16 ES636 True RMS-to-DC Converters The dB output is obtained by the voltage at the emitter of Q3, which is proportional to the -log VIN. The emitter follower Q5 buffers and level shifts this voltage so that the dB output is zero when the externally set emitter current for Q5 approximates I3. V+ BUFIN 7 + A4 BUFOUT 6 I3 R1 6K 4 8 I OUT R2 10K RL 9 Absolute value/voltage-current converter i upper R4 20K CAV I4 I1 A3 Vin R4 Q1 Q3 5 dB OUT + Vin 1 + A1 i lower 8K + A2 Q2 Q4 Q5 R3 10K 8K One-Quadrant square/divider V- Figure 1. ES636 Simplified Schematic Standard Connection (Figure 2) The standard RMS connection requires only one external component, CAV. In this configuration the ES636 measures the RMS of the AC and DC levels present at the input, but shows an error for low-frequency inputs as a function of the CAV filter capacitor. Figure 3 gives practical values of CAV for various values of averaging error over frequency for the standard RMS connections (no post filtering). If a 3uF capacitor is chosen, the additional error at 30Hz will be 1%. If the DC error can be rejected, a capacitor should be connected in series with the input, as would typically be the case in single-supply operation. The input and output signal ranges are a function of the supply voltages. Refer to the electrical characteristics for guaranteed performance. The buffer amplifier can be used either for lowering the output impedance of the circuit, or for other applications such as buffering high-impedance input signals. The ES636 can be used in current output mode by disconnecting the internal load resistor, RL, from ground. The current output is available at pin 8 with a nominal scale of 100μA/ VRMS input for the ES636. The 7 09/02/16 ES636 True RMS-to-DC Converters output is positive. - Cav + Vin 1 VIN ABSOLUTE VALUE +Vs 14 +Vs 2 Enable 13 -Vs 3 -Vs CAV SQUARE DIVIDER 12 4 11 5 dB CURRENT MIRROR COMMON 10 Vout 6 BUF out + BUF - 9 RL 8 7 BUF in Iout CF (optional) Figure 2. Standard connection for ES636. 8 09/02/16 ES636 True RMS-to-DC Converters High-Accuracy Adjustments The accuracy of the ES636 can be further improved by the external trimming scheme as shown in Figure 4. The input should be grounded and R4 adjusted to give zero output from pin 6. R1 and R2 are trimmed to give the correct value for a calibrated signal. Cav + Vin 200 Ω R1 1 VIN ABSOLUTE VALUE +Vs 14 +Vs 2 Enable 13 -Vs 3 -Vs CAV SQUARE DIVIDER 12 4 11 5 dB CURRENT MIRROR COMMON 10 R2 154 Ω 9 RL +Vs Vout 6 BUF out + BUF - 7 BUF in Iout 8 470 Ω R3 R4 500KΩ -Vs Figure 3. External Gain and Offset Trimming Circuit. Power-Down Function The ES636 provides a chip-enable pin (Pin 2). To enable the device, this pin must be connected to –Vs. If it is connected to V+, the device will enter power-down mode. The 9 09/02/16 ES636 True RMS-to-DC Converters current it draws at this mode is less than 1uA. Choosing the Averaging Time Constant The ES636 computes the RMS value of AC and DC signals. At low frequencies and DC, d the output tracks the input exactly; at higher frequencies, the average output approaches the RMS value of the input signal. The actual output differs from the ideal by an average (or DC) error plus some amount of ripple. The DC error term is a function of the value of CAV and the input signal frequency. The output ripple is inversely proportional to the value of CAV. Waveforms with high crest factors, such as a pulse train with low duty cycle, should have an average time constant chosen to be at least ten times the signal period. Using a large value of CAV to remove the output ripple increases the setting time for a step change in the input signal level. Figure 3 shows the relationship between CAV and 1 % settling time, where 110ms settling equals 4uF of CAV. The settling time, or time for the RMS converter to settle to within a given percent of the change in RMS level, is set by the averaging time constant, which varies approximately 2:1 between decreasing and increasing input signals. In addition, the settling time also varies with input signal levels, increasing as the input signal is reduced, and decreasing as the input is increased. External Av. CAP, Cav Settling Time (sec) 30uF 20uF 0.83 0.55 10uF 0.27 5uF 4uF 3uF 2uF 1% 0.5% 0.1% 0.14 0.11 0.08 0.06 1uF 10 Frequency (Hz) 100 0.03 Figure 4. Errors/Settling Time Graph for Standard Connection 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. A better method to 10 09/02/16 ES636 True RMS-to-DC Converters reduce the settling time and ripple is to use a post filter. Two suggested circuits are shown in Figure 4 and Figure 5. A post filter allows a smaller CAV. With post filter, the value of CAV should be just large enough to give the maximum dc error at the lowest frequency of interest. And the output ripple will be removed by the post filter. Vin 1 ABSOLUTE VALUE 14 +Vs Vin 1 ABSOLUTE VALUE 14 +Vs 2 13 2 13 -Vs +Vs Cav +- 3 SQUARE DIVIDER 12 -Vs +Vs Cav +- 3 SQUARE DIVIDER 12 4 11 4 11 5 CURRENT MIRROR 10 5 CURRENT MIRROR 10 Vout 6 + BUF - 9 Vout 6 + BUF - 9 7 8 7 8 + - C2 + C2 - 10K Ω C3 Figure 5(a). ES636 with a One-Pole Filter (b) with a Two-Pole Filter Decibel Output (dB) The dB output of the ES636 originates in the squarer/divider section and works well over a 50dB range. The dB output has a temperature drift of 0.03dB/℃. Frequency Response ES636 utilizes a logarithmic circuit in performing the RMS computation of the input signal. The bandwidth of the RMS converters is proportional to signal level. Figure 11 represent the frequency response of the converters from 35mV to 1V for ES636. The dashed lines indicate the upper frequency limits for 1%, 10%, and ±3dB of reading additional error. Caution must be used when designing RMS measuring systems so that 11 09/02/16 ES636 True RMS-to-DC Converters overload does not occur. The input clipping level for ES636 is ±10V. A 3VRMS signal with a crest factor of 3 has a peak input of 9V. Vout (mV) 1000 1Vrms Input 400mVrms Input 200mVrms Input 100 100mVrms Input 35mVrms Input 1% 10% 3dB 10 10k 100k 1M Frequency (Hz) 10M Figure 6. Frequency Response for ES636 12 09/02/16 ES636 True RMS-to-DC Converters Packaging 1.14 Pin SOP Package 2. Dimension Paramenters 13 09/02/16