Not Recommended for New Designs

Transcription

1 Not Recommended for New Designs This product was manufactured for Maxim by an outside wafer foundry using a process that is no longer available. It is not recommended for new designs. The data sheet remains available for existing users. A Maxim replacement or an industry second-source may be available. Please see the QuickView data sheet for this part or contact technical support for assistance. For further information, contact Maxim s Applications Tech Support.

2 -0; Rev ; / True RMS-to-DC Converters General Description The MXA and MX are true RMS-to-DC converters. They feature low power and are designed to accept low-level input signals from 0 to V RMS for the MXA and 0 to 00mV RMS for the MX. Both devices accept complex input waveforms containing AC and DC components. They can be operated from either a single supply or dual supplies. Both devices draw less than ma of quiescent supply current, making them ideal for battery-powered applications. Input and output offset, positive and negative waveform symmetry (DC reversal), and full-scale accuracy are laser trimmed, so that no external trims are required to achieve full rated accuracy. Applications Digital Multimeters Battery-Powered Instruments Panel Meters Process Control TOP VIEW COMMON R L Pin Configurations I OUT 0 MXA MXB TO-00 IN OUT Features True RMS-to-DC Conversion Computes RMS of AC and DC Signals Wide Response: MHz Bandwidth for V RMS > V (MXA) MHz Bandwidth for V RMS > 00mV (MX) Auxiliary Output: 0 Range (MXA) 0 Range (MX) Single- or Dual-Supply Operation Low Power:.mA typ (MXA) 00µA typ (MX) PART MXAJC/D MXAJCWE MXAJD MXAJH MXAJN MXAJQ* MXAKCWE MXAKD MXAKH Ordering Information TEMP. RANGE 0 C to +0 C 0 C to +0 C 0 C to +0 C PIN-PACKAGE Dice** Wide SO Ceramic 0 C to +0 C 0 TO-00 0 C to +0 C Plastic DIP 0 C to +0 C CERDIP 0 C to +0 C Wide SO 0 C to +0 C Ceramic 0 C to +0 C 0 TO-00 MXAKN 0 C to +0 C Plastic DIP MXAKQ* 0 C to +0 C CERDIP MXASD - C to + C Ceramic Ordering Information continued at end of data sheet. * Maxim reserves the right to ship ceramic packages in lieu of CERDIP packages. ** Dice are specified at T A = + C. Typical Operating Circuits MXA/MX N.C. OUT IN MXA MX DIP N.C. N.C. N.C. 0 COMMON R L I OUT Pin Configurations continued at end of data sheet. Typical Operating Circuits continued at end of data sheet. 0 Maxim Integrated Products For free samples & the latest literature: or phone For small orders, phone ext..

3 MXA/MX MAXIMUM RATINGS Supply Voltage: Dual Supplies (MXA)...±V (MX)...±V Single Supply (MXA)...+V (MX)...+V Input Voltage (MXA)...±V (MX)...±V Power Dissipation (Package) Plastic DIP (derate mw/ C above + C)...0mW Small Outline (derate 0mW/ C above + C)...00mW Ceramic (derate 0mW/ C above + C)...00mW TO-00 metal can (derate mw/ C above + C)...0mW Output Short-Circuit Duration...Indefinite Operating Temperature Ranges Commercial (J, K)...0 C to +0 C Military (S)...- C to + C Storage Temperature Range...- C to +0 C Lead Temperature (soldering, 0sec)...00 C Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS MXA (T A = + C, = +V, = -V, unless otherwise noted.) PARAMETER Transfer Equation Averaging Time Constant CONVERSION ACCURACY Total Error, Internal Trim (Note ) Total Error vs. Temperature Total Error vs. Supply Bandwidth for % Additional Error (0.0) T MIN to +0 C +0 C to + C = [avg. ( ) ] / Total Error vs. DC Reversal MXAJ, AS ±0. MXAK ±0. Total Error, External Trim MXAJ, AS ± ±0. (Note ) MXAK ± ±0. ERROR vs. CREST FACTOR (Note ) Crest Factor to Specified Accuracy Additional Error Crest Factor = -0. Crest Factor = -.0 FREQUENCY RESPONSE (Note ) Figure MXAJ, AS MXAK = 0mV = 00mV = V = 0mV MIN TYP MAX ±0. ±0.0 ± Bandwidth = 00mV 0 = V CONDITIONS MXAJ MXAK MXAS MXAS khz 0 0. ± ±0. ± ±0. ±0. ±0.0 ±0.0 ±0.00 ±0. ±0.00 ±0.0 ±0.00 UNITS ms/µf / C /V % of % of khz MHz

4 ELECTRICAL CHARACTERISTICS MXA (continued) (T A = + C, = +V, = -V, unless otherwise noted.) PARAMETER INPUT CHARACTERISTICS Input Signal Range CONDITIONS MIN TYP MAX UNITS ±V Supplies 0 to V RMS Continuous RMS Peak Transient ±0 V PK ±V Supplies 0 to V RMS Continuous RMS Peak Transient ± V PK Safe Input All Supplies ± V PK Input Resistance kω Input Offset Voltage OUTPUT CHARACTERISTICS Offset Voltage MXAJ, AS 0. ± MXAK 0. ± T A = + C MXAJ ± ± MXAK ±0. ± MXAS ± MXAJ, AK ±0. T A = T MIN to T MAX MXAS ±0. mv mv mv/ C MXA/MX Output Voltage Swing Output Current Supply Voltage MXAJ, AK ±0. MXAS ±0. ±V Supplies 0 to. ±V Supplies 0 to Source ma Sink -0 µa Short Circuit Current 0 ma Output Resistance 0. Ω OUTPUT Error MXAJ ±0. ±0. = mv to V RMS, MXAK ±0. ±0. 0 = V RMS MXAS ±0. ±0. Scale Factor - mv/ Scale Factor TC (Uncompensated) 0. mv/v V % of / C I REF 0 = V RMS 0 0 µa I REF Range 00 µa I OUT TERMINAL I OUT Scale Factor 0 µa/v RMS I OUT Scale Factor Tolerance ±0 ±0 % Output Resistance 0 0 kω Voltage Compliance to ( -.) V

5 MXA/MX ELECTRICAL CHARACTERISTICS MXA (continued) (T A = + C, = +V, = -V, unless otherwise noted.) Input Bias Current 0 00 na Input Resistance 0 Ω Output Current PARAMETER FER AMPLIFIER Input and Output Voltage Range Input Offset Voltage R S = kω CONDITIONS MIN TYP MAX to ( -.) Source + ma Sink -0 µa Short-Circuit Current 0 ma Small-Signal Bandwidth UNITS ±0. ± mv MHz Slew Rate (Note ) V/µs V ELECTRICAL CHARACTERISTICS MX (T A = + C, = +V, = -V, unless otherwise noted.) PARAMETER Transfer Equation Averaging Time Constant CONVERSION ACCURACY Total Error, Internal Trim (Notes, ) Total Error vs. Temperature (0 C to +0 C) Total Error vs. Supply Figure MXJ MXK MXJ MXK Total Error vs. DC Reversal = 00mV MXJ ±0. MXK ±0. Total Error, External Trim MXJ ±0. ±0. (Note ) MXK ±0. ±0. ERROR vs. CREST FACTOR (Note ) Crest Factor to Specified Accuracy Additional Error Crest Factor = -0. Crest Factor = -0. FREQUENCY RESPONSE (Notes, ) = 0mV Bandwidth for % V 0 Additional Error (0.0) IN = 00mV = 00mV 0 = 0mV ± Bandwidth = 00mV 00 = 00mV CONDITIONS MIN TYP MAX = [avg. ( )] / ±0. ± ±0. ±.0 ±0. ±0. ±0. ±0.0 ±0. ±0.00 UNITS ms/µf / C /V ±% of ±% of khz khz MHz

6 ELECTRICAL CHARACTERISTICS MX (continued) (T A = + C, = +V, = -V, unless otherwise noted.) Input Signal Range Safe Input All Supplies ± V PK Input Resistance...00 kω Input Offset Voltage I REF I REF Range PARAMETER INPUT CHARACTERISTICS ±. MXJ ±0. MXK ±0. OUTPUT CHARACTERISTICS (Note ) MXJ ±0. T A = + C MXK ±0. Offset Voltage T A = T MIN to T MAX ±0 With Supply Voltage ±0. +V, upplies 0 to Output Voltage Swing ±V to ±.V Supplies 0 to. Output Resistance 0 kω OUTPUT Error I OUT Scale Factor I OUT Scale Factor Tolerance mv 00mV CONDITIONS MIN TYP MAX ± ± MXJ ±0. ±0. MXK ±0. ±0. 0 UNITS V PK mv mv µv/ C mv/v Scale Factor - mv/ Scale Factor Tempco I OUT TERMINAL Output Resistance Voltage Compliance FER AMPLIFIER Input and Output Voltage Range Input Offset Voltage Continuous RMS, All Supplies +V, upplies Peak Transient ±.V Supplies ±V Supplies R S = 0kΩ 0 to %/ C -0.0 / C µa µa 00 µa/v RMS -0 ±0 +0 % 0 kω to ( - ) to ( -.0) ±0. ± ±0. ± Input Current na Input Resistance 0 Ω Output Current 0 = V RMS MXJ MXK mv RMS Source + ma Sink -0 µa Short-Circuit Current 0 ma Small-Signal Bandwidth MHz Slew Rate (Note ) V/µs V V V mv MXA/MX

7 MXA/MX ELECTRICAL CHARACTERISTICS MX (continued) (T A = + C, = +V, = -V, unless otherwise noted.) PARAMETER POWER SUPPLY CONDITIONS Rated Performance +/- V Dual Supplies +/-. ±. V Single Supply + + V Quiescent Current (Note 0) 0. ma Detailed Description The MXA/MX uses an implicit method of RMS computation that overcomes the dynamic range as well as other limitations inherent in a straightforward computation of the RMS. The actual computation performed by the MXA/MX follows the equation: V RMS = Avg. [VIN /V RMS ] The input voltage,, applied to the MXA/MX is processed by an absolute-value/voltage to current converter that produces a unipolar current I (Figure ). This current drives one input of a squarer/divider that produces a current I that has a transfer function: I = I I The current I drives the internal current mirror through a lowpass filter formed by R and an external capacitor, CAV. As long as the time constant of this filter is greater than the longest period of the input signal, I is averaged. The current mirror returns a current, I, to the square/divider to complete the circuit. The current I is then a function of the average of (I /I ), which is equal to I RMS. The current mirror also produces a I output current, I OUT, that can be used directly or converted to a voltage using resistor R and the internal buffer to provide a low-impedance voltage output. The transfer function for the MXA/MX is: = R I RMS = MIN TYP MAX Note : Accuracy is specified for 0 to V RMS, DC or khz sine-wave input with the MXA connected as in Figure. Note : Error vs. crest factor is specified as an additional error for V RMS rectangular pulse stream, pulse width = 00µs. Note : Input voltages are expressed in volts RMS, and error as % of reading. Note : With kω external pull-down resistor. Note : Accuracy is specified for 0 to 00mV, DC or khz sine-wave input. Accuracy is degraded at higher RMS signal levels. Note : Measured at pin of DIP and SO (I OUT ), with pin tied to COMMON. Note : Error vs. crest factor is specified as an additional error for 00mV RMS rectangular pulse input, pulse width = 00µs. Note : Input voltages are expressed in volts RMS. Note : With 0kΩ external pull-down resistor from pin ( OUT) to. Note 0: With input tied to COMMON. UNITS The output is obtained by the voltage at the emitter of Q, which is proportional to the -log VIN. The emitter follower Q buffers and level shifts this voltage so that the output is zero when the externally set emitter current for Q approximates I. Standard Connection (Figure ) The standard RMS connection requires only one external component,. In this configuration the MXA/MX measures the RMS of the AC and DC levels present at the input, but shows an error for lowfrequency inputs as a function of the filter capacitor. Figure gives practical values of for various values of averaging error over frequency for the standard RMS connections (no post filtering). If a µf capacitor is chosen, the additional error at 00Hz will be %. 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 highimpedance input signals. The MXA/MX can be used in current output mode by disconnecting the internal load resistor, R L, from ground. The current output is available at pin (pin 0 on the H package) with a nominal scale of 0µA/VRMS input for the MXA and 00µA/VRMS input for the MX. The output is positive.

8 MXA / VOLTAGE- CONVERTER 0.mA F.S. A I I R k I OUT 0.mA F.S. I REF R k COM 0 R L OUT MXA/MX R 0k A R - I Q Q Q Q Q F IN A FER F OUT R k A k k ONE-QUADRANT / k Figure. MXA Simplified Schematic 0 0 MXA MX Figure. MXA/MX Standard RMS Connection

9 MXA/MX EXTERNAL AVERAGING CAP, CAV (µf) % 0.% k FREQUENCY (Hz) OUTPUT SETTLING TIME TO COMPLETE % OF STEP_ (seconds) R 0 R R R Figure. Lower Frequency for Stated % of Error and Settling Time for Circuit shown in Figure High-Accuracy Adjustments The accuracy of the MXA/MX can be improved by the addition of external trims as shown in Figure. R trims the offset. The input should be grounded and R adjusted to give zero volts output from pin. R is trimmed to give the correct value for either a calibrated DC input or a calibrated AC signal. For example: 00mV DC input should give 00mV DC output; a ±00mV peak-to-peak sine-wave should give mv DC output. Single-Supply Operation Both the MXA and the MX can be used with a single supply down to +V (Figure ). The major limitation of this connection is that only AC signals can be measured, since the differential input stage must be biased off ground for proper operation. The load resistor is necessary to provide output sink current. The input signal is coupled through C and the value chosen so that the desired low-frequency break point is obtained with the input resistance of.kω for the MXA and.kω for the MX. Figure shows how to bias pin 0 within the range of the supply voltage (pin on H packages). It is critical that no extraneous signals are coupled into this pin. A capacitor connected between pin 0 and ground is recommended. The common pin requires less than µa of input current, and if the current flowing through resistors R and R is chosen to be approximately 0 times the common pin current, or 0µA, the resistor values can easily be calculated. Choosing the Averaging Time Constant Both the MXA and MX compute the RMS value of AC and DC signals. At low frequencies and DC, the output tracks the input exactly; at higher frequencies, MXA MX R R R R MXA 00Ω Ω 0kΩ 0kΩ MX 00Ω Ω 0kΩ 00kΩ Figure. Optional External Gain and Output Offset Trims R L 0k TO k C MXA MX R R C MXA 0kΩ 0kΩ µf Figure. Single-Supply Operation MX 0kΩ kω.µf 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 and the input signal frequency. The output ripple is inverse- 0 0.µF R R 0.µF

10 SETTLING TIME RELATIVE TO VRMS INPUT SETTLING TIME MXA m 0m 00m 0 RMS INPUT LEVEL (V) SETTLING TIME RELATIVE TO 00mVRMS INPUT SETTLING TIME MX m 0m 00m RMS INPUT LEVEL (V) MXA/MX Figure a. MXA Settling Time vs. Input Level ly proportional to the value of. 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 to remove the output ripple increases the settling time for a step change in the input signal level. Figure shows the relationship between and settling time, where ms settling equals µf of. 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 : between increasing and decreasing input signals. For example, increasing input signals require. time constants to settle to within %, and. time constants for decreasing signals levels. 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 as shown in Figures a and b. Using Post Filters A post filter allows a smaller value of CAV, and reduces ripple and improves the overall settling time. The value of should be just large enough to give the maximum DC error at the lowest frequency of interest. The post filter is used to remove excess output ripple. Figures,, and give recommended filter connections and values for both the MXA and MX. Table lists the number of time constants required for the RMS section to settle to within different percentages of the final value for a step change in the input signal. Figure b. MX Settling Time vs. Input Level Table. Number of RC Time Constants (τ) Required for MXA/MX RMS Converters to Settle to Within Stated % of Final Value PARAMETERS Basic Formulas Settling Time to Within Stated % of New RMS Level % 0.% 0.0% FOR INCREASING AMPLITUDES Note: (τ) Settling Times for Linear RC Filter FOR DECREASING AMPLITUDES V - e -T/RC V e -T/RC.τ/.0τ.τ/.τ.τ/.τ.τ/.τ.τ/.τ.τ/.τ Decibel Output () The output of the MXA/MX originates in the squarer/divider section and works well over a 0 range. The connection for measurements is shown in Figure 0. The output has a temperature drift of 0.0/ C, and in some applications may need to be compensated. Figure 0 shows a compensation scheme. The amplifier can be used to scale the output for a particular application. The values used in Figure 0 give an output of +00mV/.

11 MXA/MX V RMS OUT N.C. -V C S AV MXA MX 0 N.C. N.C. N.C. COMMON R L I OUT C * MXA = kω MX = 0kΩ C R X * 0 MXA MX C V RMS OUT Figure. MXA/MX with a One-Pole Output Filter Figure. MXA/MX with a Two-Pole Output Filter EDC ERROR OR RIPPLE (% OF READING) 0 DC ERROR (ALL FILTERS) PK-PK RIPPLE PK-PK RIPPLE (ONE POLE) C =.µf PK-PK RIPPLE (TWO POLE) C = C =.µf k 0k MXA ONE-POLE FILTER C.µF µf TWO-POLE FILTER C C C AF.µF.µF µf FREQUENCY (Hz) MX.µF µf.µf.µf µf R X = 0 Frequency Response The MXA/MX utilizes a logarithmic circuit in performing the RMS computation of the input signal. The bandwidth of the RMS converters is proportional to signal level. Figures and represent the frequency response of the converters from 0mV to VRMS for the MXA and mv to V for the MX, respectively. The dashed lines indicate the upper frequency limits for %, 0%, and ± of reading additional error. Caution must be used when designing RMS measuring systems so that overload does not occur. The input clipping level for the MX is ±V, and for the MXA it is ±0V. A VRMS signal with a crest factor of has a peak input of V. Application in a Low-Cost DVM A low-cost digital voltmeter (DVM) using just two integrated circuits plus supporting circuitry and LCD display is shown in Figure. The MAX0 is a / digit integrating A/D converter with precision bandgap reference. The 0MΩ input attenuator is AC coupled to pin of the MX buffer amplifier. The output from the MX is connected to the MAX0 to give a direct reading to the LCD display. Figure. Performance Features of Various Filter Types for MXA/MX 0

12 OUT -mv/ C C 0.µF MXA MX 0 ZERO LINEAR RMS OUTPUT.V R R MX0J GROUND.V TO V R k* R 00Ω GAIN R k MAX00 COMPENSATED OUT +0.V/ MXA/MX *SPECIAL TC COMP RESISTOR: +00PPM, k, % Figure 0. Connection VOUT (V) V RMS INPUT % 0% V RMS INPUT 00mV RMS INPUT 0mV RMS INPUT ± VOUT (V) 00m 00m 0m 0m m V RMS INPUT 00mV RMS INPUT 00mV RMS INPUT 0mV RMS INPUT 0mV RMS INPUT V RMS INPUT % 0% ± k 0k 00k M 0M FREQUENCY (Hz) 00µ k 0k 00k M 0M FREQUENCY (Hz) Figure. MXA High-Frequency Response Figure. MX High-Frequency Response

13 MXA/MX R M R 00k R 0k R 0k 00mV V 0V 00V COM D IN C 0.0µF R k W 0% D IN R M.µF R 0k C.µF 0k 0k MX 0 C.µF R R k 00k 0 SET R0 0k LIN LIN R k LIN SCALE D D D R 00Ω R M R 0k SCALE N LIN C 0.0µF +V DD DIGIT ADC MAX0 REF HI REF LO COM IN HI IN LO V+ V- V BATTERY DIGIT LCD DISPLAY Figure. Portable High-Z Input RMS DPM and Meter Typical Operating Circuits (continued) 0 TOP VIEW Pin Configurations (continued) N.C. OUT IN N.C. MXA MX SO N.C. N.C. N.C. COMMON R L 0 I OUT N.C. Ordering Information (continued) PART MXASH TEMP. RANGE - C to + C PIN-PACKAGE 0 TO-00 MXASQ* - C to + C CERDIP MXJC/D MXJCWE MXJD 0 C to +0 C Dice** 0 C to +0 C Wide SO 0 C to +0 C Ceramic MXJH 0 C to +0 C 0 TO-00 MXJN 0 C to +0 C Plastic DIP * Maxim reserves the right to ship ceramic packages in lieu of CERDIP packages. ** Dice are specified at T A = + C. PART TEMP. RANGE PIN-PACKAGE MXJQ* MXKCWE MXKD MXKH 0 C to +0 C CERDIP 0 C to +0 C Wide SO 0 C to +0 C Ceramic 0 C to +0 C 0 TO-00 MXKN 0 C to +0 C Plastic DIP MXKQ* 0 C to +0 C CERDIP Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. Maxim Integrated Products, 0 San Gabriel Drive, Sunnyvale, CA Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.