MX636JN中文资料

General Description

The MX536A and MX636 are true RMS-to-DC convert-ers. They feature low power and are designed to accept low-level input signals from 0 to 7V RMS for the MX536A and 0 to 200mV RMS for the MX636. Both devices accept complex input waveforms containing AC and DC com-ponents. They can be operated from either a single sup-ply or dual supplies. Both devices draw less than 1mA of quiescent supply current, making them ideal for bat-tery-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

____________________________Features

o True RMS-to-DC Conversion

o Computes RMS of AC and DC Signals o Wide Response:

2MHz Bandwidth for V RMS > 1V (MX536A)1MHz Bandwidth for V RMS > 100mV (MX636)o Auxiliary dB Output:60dB Range (MX536A)

50dB Range (MX636)

o Single- or Dual-Supply Operation o Low Power: 1.2mA typ (MX536A)

800μA typ (MX636)

MX536A/MX636

True RMS-to-DC Converters

________________________________________________________________Maxim Integrated Products

1

Pin Configurations

_________Typical Operating Circuits

19-0824; Rev 2; 3/96

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 = +25°C.

For free samples & the latest literature: https://www.sodocs.net/doc/5617871849.html,, or phone 1-800-998-8800.For small orders, phone 408-737-7600 ext. 3468.

M X 536A /M X 636

True RMS-to-DC Converters 2_______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS—MX536A

(T A = +25°C, +V S = +15V, -V S = -15V, unless otherwise noted.)

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.

Supply Voltage:Dual Supplies (MX536A) (18)

(MX636) (12)

Single Supply (MX536A) (36)

(MX636) (24)

Input Voltage (MX536A) (25)

(MX636) (12)

Power Dissipation (Package)

Plastic DIP (derate 12mW/°C above +75°C)...............450mW Small Outline (derate 10mW/°C above +75°C)............400mW Ceramic (derate 10mW/°C above +75°C)...................500mW TO-100 metal can (derate 7mW/°C above +75°C)......450mW

Output Short-Circuit Duration........................................Indefinite Operating Temperature Ranges

Commercial (J, K)...............................................0°C to +70°C Military (S)......................................................-55°C to +125°C Storage Temperature Range.............................-55°C to +150°C Lead Temperature (soldering, 10sec)................................300°C

MX536A/MX636

True RMS-to-DC Converters

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ELECTRICAL CHARACTERISTICS—MX536A (continued)

(T A = +25°C, +V S = +15V, -V S = -15V, unless otherwise noted.)

M X 536A /M X 636

True RMS-to-DC Converters 4_______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS—MX536A (continued)

(T A = +25°C, +V S = +15V, -V S = -15V, unless otherwise noted.)

ELECTRICAL CHARACTERISTICS—MX636

(T A = +25°C, +V S = +3V, -V S = -5V, unless otherwise noted.)

MX536A/MX636

True RMS-to-DC Converters

_______________________________________________________________________________________

5

ELECTRICAL CHARACTERISTICS—MX636 (continued)

(T

= +25°C, +V = +3V, -V = -5V, unless otherwise noted.)

M X 536A /M X 636

_______________Detailed Description

The MX536A/MX636 uses an implicit method of RMS computation that overcomes the dynamic range as well as other limitations inherent in a straightforward compu-tation of the RMS. The actual computation performed by the MX536A/MX636 follows the equation:

V RMS = Avg. [V IN 2/V RMS ]

The input voltage, V IN , applied to the MX536A/MX636 is processed by an absolute-value/voltage to current con-verter that produces a unipolar current I 1(Figure 1).This current drives one input of a squarer/divider that produces a current I 4that has a transfer function:

I 4= I 1

2I 3The current I 4drives the internal current mirror through a lowpass filter formed by R1 and an external capaci-tor, C AV . As long as the time constant of this filter is greater than the longest period of the input signal, I 4is averaged. The current mirror returns a current, I 3, to the square/divider to complete the circuit. The current I 4is then a function of the average of (I 12/I 4), which is equal to I 1RMS .

The current mirror also produces a 2 · I 4output current,I OUT , that can be used directly or converted to a volt-age using resistor R2 and the internal buffer to provide a low-impedance voltage output. The transfer function for the MX536A/MX636 is:

V OUT = 2 · R2 · I RMS = V IN

The dB output is obtained by the voltage at the emitter of Q3, which is proportional to the -log V IN . 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 I 3.

Standard Connection

(Figure 2)

The standard RMS connection requires only one exter-nal component, C AV . In this configuration the MX536A/MX636 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 C AV filter capaci-tor. Figure 3 gives practical values of C AV for various values of averaging error over frequency for the stan-dard RMS connections (no post filtering). If a 3μF capacitor is chosen, the additional error at 100Hz 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 cir-cuit, or for other applications such as buffering high-impedance input signals. The MX536A/MX636 can be used in current output mode by disconnecting the inter-nal load resistor, R L , from ground. The current output is available at pin 8 (pin 10 on the “H” package) with a nominal scale of 40μA/V RMS input for the MX536A and 100μA/V RMS input for the MX636. The output is positive.

True RMS-to-DC Converters 6_______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS—MX636 (continued)

(T

= +25°C, +V = +3V, -V = -5V, unless otherwise noted.)Note 1:Accuracy is specified for 0 to 7V RMS , DC or 1kHz sine-wave input with the MX536A connected as in Figure 2.Note 2:Error vs. crest factor is specified as an additional error for 1V RMS rectangular pulse stream, pulse width = 200μs.Note 3:Input voltages are expressed in volts RMS, and error as % of reading.Note 4:With 2k ?external pull-down resistor.

Note 5:Accuracy is specified for 0 to 200mV, DC or 1kHz sine-wave input. Accuracy is degraded at higher RMS signal levels.Note 6:Measured at pin 8 of DIP and SO (I OUT ), with pin 9 tied to COMMON.

Note 7:Error vs. crest factor is specified as an additional error for 200mV RMS rectangular pulse input, pulse width = 200μs.Note 8:Input voltages are expressed in volts RMS.

Note 9:With 10k ?external pull-down resistor from pin 6 (BUF OUT) to -V S .Note 10:With BUF input tied to COMMON.

MX536A/MX636

True RMS-to-DC Converters

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7

Figure 1. MX536A Simplified Schematic

Figure 2. MX536A/MX636 Standard RMS Connection

M X 536A /M X 636

High-Accuracy Adjustments

The accuracy of the MX536A/MX636 can be improved by the addition of external trims as shown in Figure 4.R4 trims the offset. The input should be grounded and R4 adjusted to give zero volts output from pin 6. R1 is trimmed to give the correct value for either a calibrated DC input or a calibrated AC signal. For example: 200mV DC input should give 200mV DC output; a ±200mV peak-to-peak sine-wave should give 141mV DC output.

Single-Supply Operation

Both the MX536A and the MX636 can be used with a single supply down to +5V (Figure 5). The major limita-tion 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 resis-tor is necessary to provide output sink current. The input signal is coupled through C2 and the value cho-sen so that the desired low-frequency break point is obtained with the input resistance of 16.7k ?for the MX536A and 6.7k ?for the MX636.

Figure 5 shows how to bias pin 10 within the range of the supply voltage (pin 2 on “H” packages). It is critical that no extraneous signals are coupled into this pin. A capacitor connected between pin 10 and ground is recommended. The common pin requires less than 5μA of input current, and if the current flowing through resis-tors R1 and R2 is chosen to be approximately 10 times the common pin current, or 50μA, the resistor values can easily be calculated.

Choosing the Averaging Time Constant

Both the MX536A and MX636 compute 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 C AV and the input signal frequency. The output ripple is inverse-

True RMS-to-DC Converters 8

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Figure 3. Lower Frequency for Stated % of Reading Error and Settling Time for Circuit shown in Figure 2

Figure 4. Optional External Gain and Output Offset Trims

Figure 5. Single-Supply Operation

ly proportional to the value of C AV . 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 C AV to remove the output ripple increases the settling time for a step change in the input signal level. Figure 3 shows the relationship between C AV and settling time, where 115ms settling equals 1μF of C AV . 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 increasing and decreasing input signals. For example, increasing input signals require 2.3 time constants to settle to within 1%, and 4.6time 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 6a and 6b.

Using Post Filters

A post filter allows a smaller value of C AV , and reduces ripple and improves the overall settling time. The value of C AV should be just large enough to give the maxi-mum DC error at the lowest frequency of interest. The post filter is used to remove excess output ripple.Figures 7, 8, and 9 give recommended filter connec-tions and values for both the MX536A and MX636.Table 1 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.

Decibel Output (dB)

The dB output of the MX536A/MX636 originates in the squarer/divider section and works well over a 60dB range. The connection for dB measurements is shown in Figure 10. The dB output has a temperature drift of 0.03dB/°C, and in some applications may need to be compensated. Figure 10 shows a compensation scheme. The amplifier can be used to scale the output for a particular application. The values used in Figure 10 give an output of +100mV/dB.

MX536A/MX636

True RMS-to-DC Converters

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10

012.51m

100m

10

1

57.5RMS INPUT LEVEL (V)

S E T T L I N G T I M E R E L A T I V E T O 1V R M S I N P U T S E T T L I N G T I M E

10m

MX536A

Figure 6a. MX536A Settling Time vs. Input Level 10

012.51m

100m

1

5

7.5

RMS INPUT LEVEL (V)

S E T T L I N G T I M E R E L A T I V E T O 200m V R M S I N P U T S E T T L I N G T I M E

10m

MX636

Figure 6b. MX636 Settling Time vs. Input Level

Note:(τ) Settling Times for Linear RC Filter

M X 536A /M X 636

Frequency Response

The MX536A/MX636 utilizes a logarithmic circuit in per-forming the RMS computation of the input signal. The bandwidth of the RMS converters is proportional to sig-nal level. Figures 11 and 12 represent the frequency response of the converters from 10mV to 7V RMS for the MX536A and 1mV to 1V for the MX636, respectively.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 overload does not occur. The input clipping level for the MX636 is ±12V, and for the MX536A it is ±20V. A 7V RMS signal with a crest factor of 3 has a peak input of 21V.

Application in a Low-Cost DVM

A low-cost digital voltmeter (DVM) using just two inte-grated circuits plus supporting circuitry and LCD dis-play is shown in Figure 13. The MAX130 is a 3 1/2 digit integrating A/D converter with precision bandgap refer-ence. The 10M ?input attenuator is AC coupled to pin 6 of the MX636 buffer amplifier. The output from the MX636 is connected to the MAX130 to give a direct reading to the LCD display.

True RMS-to-DC Converters 10

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Figure 7. MX536A/MX636 with a One-Pole Output Filter

Figure 8. MX536A/MX636 with a Two-Pole Output Filter

Figure 9. Performance Features of Various Filter Types for MX536A/MX636

MX536A/MX636

True RMS-to-DC Converters

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Figure 10. dB Connection

Figure 12. MX636 High-Frequency Response

Figure 11. MX536A High-Frequency Response

*

** Dice are specified at T A = +25°C.

M X 53

6A /M X 636

True RMS-to-DC Converters Pin Configurations (continued)

Figure 13. Portable High-Z Input RMS DPM and dB Meter

Typical Operating

________________Circuits (continued)

___________________________________________Ordering Information (continued)

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.

12____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600?1998 Maxim Integrated Products

Printed USA

is a registered trademark of Maxim Integrated Products.