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: 600μA typically Wide power supply range : from ± 2.5V to ±10V Application * Digital Multi-Meters * Battery-Powered Instruments * Panel Meter 8-pin SOP package 1 09/07/08 ES7 True RMS-to-DC Converters Pin Assignment: ES7 1 RL COM 8 ES7 2 Vin 3 PwrDown 4 -Vs +Vs 7 Vout 6 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 2 09/07/08 ES7 True RMS-to-DC Converters Absolute Maximum Ratings ......…………………………….….... ±10V Supply Voltage: Dual Supplies 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 CONDITIONS MIN Transfer Equation TYP VOUT = Averaging Time Constant MAX UNITS 2 avg.[(VIN) ] ms/μF CAV 6 CONVERSION ACCURACY Total Error, Internal Trim (Notes 1) ES7 ±0.5 ± 1.0 mV ±% of Reading Total Error vs. Temperature (0 ℃ to + 70℃) ±0.1 ±0.01 mV ±% of Reading/℃ Total Error vs. Supply ±0.1 ±0.01 mV ±% of Reading/V Total Error vs. DC Reversal VIN=+400mV ±2.0 Crest Factor = 1 Crest Factor = 2 Additional Error (Note 2) Cav=2.2μF Crest Factor = 3 Crest Factor = 4 400mV Specified Accuracy 200mV 400mV 200mV 400mV 200mV 400mV 1.00 1.10 1.25 1.50 1.50 2.00 ±% of Reading ±% of Reading FREQUENCY RESPONSE Bandwidth for 1% Additional Error (0.09dB) ±3dB Bandwidth 35mV 50 100mV 200 200mV 200 400mV 200 35mV 1.0 100mV 1.0 200mV 1.0 400mV 0.5 3 kHz MHz 09/07/08 ES7 True RMS-to-DC Converters Electrical Characteristics-ES7 (continued) (TA= +25℃, Vs = +3V, -Vs = -3V, unless otherwise noted.) PARAMETER CONDITIONS MIN TYP MAX UNITS INPUT CHARACTERISTICS Input Signal range Continuous RMS, All Supplies ±2.5V Supplies Peak Transient ±3V Supplies 0 to 400 ±5V Supplies Input Resistance Input Offset Voltage (Note3) mVRMS 1 6 8 ES7 VPK 1.5 2.8 10 MΩ ±0.5 mV OUTPUT CHARACTERISTICS Output Voltage Swing +3V, -3V Supplies ±5V to ±10V Supplies 1 1 8 Output Resistance Power SUPPLY Rated Performance 1.5 10 VRMS 12 ±3 kΩ V Dual Supplies ±2.5 ±10 V Single Supply +5 +20 V 600 800 μA 60 75 μA Supply Current ±3V Supply. Vin connects to COM Supply Current (Power Down) Pin3 connects to V+ 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 = 200μs Note 3: The input offset voltage can be reduced or canceled by an external 500kohm variable resistor shown in Figure 3. 4 09/07/08 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. [VIN2／VRMS] 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: Ι12 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 Ι4 = 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 ES7 is: VOUT = 2．R2．IRMS = VIN 5 09/07/08 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 COM 8 SW1 Vin + 4.7μ Absolute Value 2 Vin +Vs 7 Ccp Rin CF (Optional) 4.7μ Square Divider 47k 3 PwrDown Vout Vout 6 Current Mirror 4 -Vs Cav 5 Cav + 2.2μ -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. 6 09/07/08 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 +Vs 1k 10 1 RL SW1 Absolute Value + 4.7μ Vin COM 8 1 2 Vin +Vs 7 Ccp Rin CF (Optional) 4.7μ Square Divider 47k 3 PwrDown2 Current Mirror 4 -Vs Vout Vout 6 Cav 5 Cav + 2.2μ -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. 7 09/07/08 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. 8 09/07/08 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 ±1% ±3db 35mV 50KHz 1MHz 100mV 200KHz 1MHz 200mV 200KHz 1MHz 400mV 200KHz 500KHZ RMS 4 Table 1 9 09/07/08 ES7 True RMS-to-DC Converters Packaging 8 Pin SOP Package Dimension Parameters 10 09/07/08