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MX636JD Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MX636JD Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 12 page 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 Ω for the MX536A and 6.7k Ω 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 CAV and the input signal frequency. The output ripple is inverse- True RMS-to-DC Converters 8 _______________________________________________________________________________________ 100 0.1 0.22 0.65 1 100 1k 1 10 10 0.01 0.1 1 FREQUENCY (Hz) 10 60 1% 0.1% Figure 3. Lower Frequency for Stated % of Reading Error and Settling Time for Circuit shown in Figure 2 14 13 12 11 10 9 8 1 2 3 4 5 6 7 ABSOLUTE VALUE SQUARER DIVIDER CURRENT MIRROR BUF VIN -VS R1 +VS VOUT CAV R2 R3 R4 -VS +VS MX536A 500 Ω 365 Ω 750k Ω 50k Ω MX636 200 Ω 154 Ω 470k Ω 500k Ω R1 R2 R3 R4 MX536A MX636 Figure 4. Optional External Gain and Output Offset Trims 14 13 12 11 10 9 8 1 2 3 4 5 6 7 ABSOLUTE VALUE SQUARER DIVIDER CURRENT MIRROR BUF VIN +VS RL VOUT CAV R1 R2 MX536A 20k Ω 10k Ω 1 µF MX636 20k Ω 39k Ω 3.3 µF R1 R2 C2 10k TO 1k 0.1 µF 0.1 µF C2 MX536A MX636 Figure 5. Single-Supply Operation |
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