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Circuit allows sample-and-hold amplifier test

Posted: 11 Jul 2011 ?? ?Print Version ?Bookmark and Share

Keywords:sampling circuit? digitizing? ADC?

This equation ensures that the difference in values between the turn-on and turn-off times of the sample-and-hold amplifier's internal analog switch won't affect the accuracy of the precision 25 and 75% duty cycles by more than 1%.

If you evaluate the equation for a high-performance analog switch, such as the Analog Devices ADG1213, you get a repetition rate of 33kHz or less. The difference due to voltage drop prevails at low-value repetition rates. In this case, the repetition rate can be the value of the frequency at which Circuit lets you test sample-and-hold amplifiers vout overbar每VSTAT≒1/10℅VINMAX, where VINMAX is the maximum input-voltage range. The best way to determine the lower limit of the repetition rate is through experimentation.

A tested sample-and-hold amplifier using the circuit in figure 2 uses a supply voltage of 每1V, a drain-to-drain voltage of 5V, and a supply voltage of 3.3V for logic circuits in the pulse generator. Two sets of measurements at 25, 75, and 100% duty-cycle values for the AGD1213's internal switch control used input voltages of 0 and 2.5V. You will measure the output-voltage difference, approximately 每0.0366 mV, and the pedestal voltage, approximately 每0.0333 mV, at a repetition rate of 1.762kHz. The value of the residual effective charge injection, QINJ, into the hold capacitor, CH=2 nF, is QINJ=CH℅VINJ.

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Figure 2: Use a digital voltmeter to measure the difference between the output and input voltages of a sample-and-hold amplifier.

The value is negative and doesn't exceed 每75 fC. The following equation defines the difference of charge injection within the 2.5V range of input voltage: 忖QINJ=QINJ(2.5V) 每 QINJ(0V) and yields a value of 每6.7 fC. The following equation determines the residual effective leakage current from the acquired values of peak voltage drop at a repetition rate of 160Hz: ILEAK=CH℅VDROPPEAK℅fREP, where ILEAK is the leakage current. A leakage current at the input voltage of 0V is approximately 17 pA, and a leakage current at the input voltage of 2.5V is approximately 每17 pA.

- Mari芍n ?tofka
??Slovak University of Technology

Reference
"Low Capacitance, Low Charge Injection, ㊣15 V/+12 V iCMOSTM Quad SPST Switches," Analog Devices Inc, 2005.

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