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ES7
True RMS-to-DC Converters
Features
* *
True RMS-to-DC Conversion Input level is specified up to 400mVRMS
Description
The ES7 series are designed for the true RMS-to-DC conversion. ES7 accept low-level input signals from 0 to 400 mV RMS complex input waveforms. ES7 can be operated form either a single supply or dual supplies. The device draw 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.
* *
Averaging capacitor is typically 2.2uF Positive output voltage * Computes RMS of AC and DC Signals * * * * * * Single or Dual Supply Operation Low Cost Power-Down Function Low Power: 600A typically Wide power supply range : from 2.5V to 10V 8-pin SOP package
Application
* Digital Multi-Meters * Battery-Powered Instruments * Panel Meter
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ES7
True RMS-to-DC Converters
Pin Assignment: ES7
1 RL
COM 8
ES7
2
Vin
+Vs 7
3
PwrDown
Vout 6
4
-Vs
Cav 5
SOP 8 Pin Package Pin Description
Pin No Symbol Type 1 RL 2 Vin I 3 PwrDown I 4 -Vs P 5 Cav I/O 6 Vout O 7 +Vs P 8 COM P I: input, O: output, P: power Description RL terminal. For zero-offset removing. Measurement input. Pull high (+Vs) to enable power-down function. Negative supply voltage. Averaging capacitor Measurement output. Positive supply voltage. Power ground
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ES7
True RMS-to-DC Converters
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
Electrical Characteristics-ES7
(TA= +25, Vs = +3V, -Vs = -3V, unless otherwise noted.)
PARAMETER Transfer Equation Averaging Time Constant CONVERSION ACCURACY Total Error, Internal Trim (Notes 1) Total Error vs. Temperature (0 to + 70) Total Error vs. Supply Total Error vs. DC Reversal VIN=+400mV Crest Factor = 1 Crest Factor = 2 Additional Error (Note 2) Cav=2.2F Crest Factor = 3 Crest Factor = 4 FREQUENCY RESPONSE 35mV Bandwidth for 1% Additional Error (0.09dB) 100mV 200mV 400mV 35mV 3dB Bandwidth 100mV 200mV 400mV 50 200 200 200 1.0 1.0 1.0 0.5 MHz kHz 400mV 200mV 400mV 200mV 400mV 200mV 400mV mV % of Reading mV % of Reading/ mV % of Reading/V 2.0 Specified Accuracy 1.00 1.10 1.25 1.50 1.50 2.00 % of Reading % of Reading CONDITIONS MIN VOUT = TYP 6 MAX
2
UNITS ms/F CAV
avg.[(VIN) ]
ES7
0.5 1.0
0.1 0.01 0.1 0.01
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ES7
True RMS-to-DC Converters
Electrical Characteristics-ES7 (continued)
(TA= +25, Vs = +3V, -Vs = -3V, unless otherwise noted.)
PARAMETER INPUT CHARACTERISTICS Continuous RMS, All Supplies 2.5V Supplies Peak Transient 3V Supplies 5V Supplies Input Resistance Input Offset Voltage (Note3) OUTPUT CHARACTERISTICS Output Voltage Swing Output Resistance Power SUPPLY Rated Performance Dual Supplies Single Supply Supply Current 3V Supply. Vin connects to COM Supply Current (Power Down) Pin3 connects to V+ 2.5 +5 600 60 +3V, -3V Supplies 5V to 10V Supplies 1 1 8 1.5 10 3 10 +20 800 75 VRMS 12 k V V V A A ES7 6 8 0 to 400 1 1.5 2.8 10 0.5 VPK M mV mVRMS CONDITIONS MIN TYP MAX UNITS
Input Signal range
Note 1: Accuracy is specified for 0 to 400mV, 1kHz sine-wave input. Accuracy is degraded at higher RMS signal levels. Note 2: Error vs. crest factor is specified as an additional error for 200mVRMS and 400mVRMS rectangular pulse input, pulse width = 200s Note 3: The input offset voltage can be reduced or canceled by an external 500kohm variable resistor shown in Figure 3.
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ES7
True RMS-to-DC Converters
Detailed Description
Figure 1 shows the simplified schematic of ES7. It consists of four major sub-circuits: absolute value circuit (rectifier), square/divider, current mirror and buffer amplifier. The actual computation performed by the ES7 follows the equation: VRMS = Avg. [VIN2VRMS] The input voltage, VIN, applied to the ES7 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:
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
4 =
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 ES7 is: VOUT = 2R2IRMS = VIN
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ES7
True RMS-to-DC Converters
Standard Connection for ES7 (Figure 2)
The standard RMS connection requires only two external components, Rin and Cav. Other components shown in figure 2 are optional. In this configuration, ES7 measure 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 4 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 Ccp should be connected in series with the input, as would typically be the case in single-supply operation.
+Vs
1 RL SW1 Vin 4.7 Ccp Rin 47k 3 PwrDown
Current Mirror Square Divider
+
COM 8
Absolute Value
2 Vin
+Vs 7
CF (Optional) 4.7 Vout
Cav +
Vout 6
4 -Vs
Cav 5
2.2 -Vs
Figure 2. Standard connection for ES7. Note: 1. SW1 is opened for AC-coupled operation, or closed for direct input. 2. PwrDown pin is pulled to -Vs or keeps floating for normal operation. Connect it to +Vs will force ES7 to enter power down mode.
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ES7
True RMS-to-DC Converters
To Adjust the zero-offset of ES7 (Figure 3)
The output of some ES7 ICs may have an offset voltage when the input is zero. The amount of this offset voltage might be different in every ES7. We provide pin1-RL to achieve the reduction of zero offset voltage. The test circuit is shown as below. The 500kohm VR, 1kohm and 10ohm resistors are used to reduce zero offset voltage. Adjusting the 500kohm VR can reduce the zero offset voltage. However it must be noted that the 10ohm resistor enlarge the output impedance. The voltage of pin6-Vout is equal to (output current)*(output impedance), so it would be enlarged too. This will cause an additional error for ES7. So we recommend that the value of resistor between pin1-RL and pin8-COM should not be too large.
500k3 -Vs 1k 10 1 RL SW1
1
+
+Vs
COM 8
Absolute Value
4.7 Vin Ccp Rin 47k
2 Vin
Square Divider
+Vs 7
CF (Optional) 4.7 Vout
Cav +
3 PwrDown2
Current Mirror
Vout 6
4 -Vs
Cav 5
2.2 -Vs Figure 3. Adjust the zero-offset Note: 1. SW1 is opened for AC-coupled operation, or closed for direct input. 2. PwrDown pin is pulled to -Vs or keeps floating for normal operation. Connect it to +Vs will force ES7 to enter power down mode. 3. The 500k ohm variable resistor can be used to adjust the zero-offset voltage.
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ES7
True RMS-to-DC Converters
Application notes
1. AC-coupled operation
Refer to the standard circuit of ES7 shown in Figure 2~3. ES7 will work in an AC-coupled operation when the SW1 is opened. In AC-coupled operation, an AC-coupled capacitor (Ccp) and bias resistors Rin must be required. For a low frequency input under 100Hz, the Ccp need a 1uF or even larger capacitor to prevent input signal from decaying. Due to the architecture of ES7, a bias current is needed to activate the input buffer. The resistor Rin applied from Vin to GND supplies a bias current flow path in AC-coupled operation. The bias current flows from GND to Vin through Rin will cause a bias voltage at Vin pin. So the Rin resistance should not be too large (cause an additional zero offset) or too small (low input impedance).
2. Power Down Function
The ES7 provides a power-down enable pin (Pin 3). To enable the device, this pin must be connected to -Vs or keep floating. If it is connected to V+, the device will enter power-down mode.
3. Post Filter CF
To reduce the output ripple of ES7, a post filter capacitor CF is required. This capacitor should be connected as shown in figure 2. 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.
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ES7
True RMS-to-DC Converters
Choosing the Averaging Time Constant
The ES7 computes the RMS value of AC and DC signals. At low frequencies and DC, 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 4 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.
Frequency Response
ES7 utilizes a logarithmic circuit in performing the RMS computation of the input signal. Table 1 represents the simplified frequency response of the converters from 35mV to 400mV for ES7. Caution must be used when designing RMS measuring systems so that overload does not occur. The input clipping level for ES7 is 10V.
Error
RMS
1% 50KHz 200KHz 200KHz 200KHz
3db 1MHz 1MHz 1MHz 500KHZ
35mV 100mV 200mV 400mV
4
Table 1
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ES7
True RMS-to-DC Converters
Packaging
8 Pin SOP Package
Dimension Parameters
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09/07/08

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