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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: 800A typical
Application
* Digital Multi-Meters * Battery-Powered Instruments * Panel Meter
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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
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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
Supply 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
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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/F CAV VOUT = [avg.(VIN)2]1/2
0.1 0.01
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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 k 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 V/ 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
k dB mV/dB %/ dB/ A A A/VRMS % k 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=10k 0.8 100 8 10 Source Sink +2 -130 20 1 3 2 300
V mV nA mA A mA MHz V/s
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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 = 200s. Note 4: Input voltages are expressed in volts RMS. Note 5: With 10 k 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. [VIN2VRMS] 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: 4 =
2 1 3
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 2I4 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=2R2IRMS= VIN
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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
IOUT
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 100A/ VRMS input for the ES636. The
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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.
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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 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 9 RL
+Vs
Vout
6
BUF out
+ BUF -
7 BUF in
Iout
8
470 R3
R4 500K
-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
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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
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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
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
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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
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ES636
True RMS-to-DC Converters
Packaging
1.14 Pin SOP Package
2. Dimension Paramenters
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