Path: EDN Asia >> Design Ideas >> IC/Board/Systems Design >> Circuit allows testing of sample-and-hold amps
IC/Board/Systems Design Share print

Circuit allows testing of sample-and-hold amps

07 Aug 2015  | Marian Stofka

Share this page with your friends

You can use the previous equations to find the values of the voltage pedestal and the peak voltage drop. A 75% duty cycle is a convenient value. The following equations are valid for this duty cycle: VINJ=6[ΔVOUT]–2/3[Δ]–16/3VSTAT, and VDROPPEAK=16[–[ΔVOUT]+1/3[Δ]+2/3VSTAT]. You must find the optimal repetition rate, fREP, of the logic-control signal. As the optimal repetition rate increases, the difference in output voltage from the input is almost purely due to dc voltage offset plus the voltage pedestal: (–VSTAT)/(VOUT–VSTAT)≈3. The following equation finds the maximum value for the optimal repetition rate: fREP≤(0.01/4)×1/(tON–tOFF), where tON and tOFF are the on and off times, respectively. This equation ensures that the difference in values between the turn-on and turn-off times of the sample-and-hold amplifier's internal analogue 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 analogue 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 –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.

Figure 2: A sample-and-hold amplifier's holding capacitor experiences a voltage drop due to leakage and bias currents plus a voltage step, which results in a difference between the amplifier's output and input voltages.

A tested sample-and-hold amplifier using the circuit in figure 2 uses a supply voltage of –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 –0.0366 mV, and the pedestal voltage, approximately –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. The value is negative and doesn't exceed –75 fC. The following equation defines the difference of charge injection within the 2.5V range of input voltage: ΔQINJ=QINJ(2.5V)–QINJ(0V) and yields a value of –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 –17 pA.

"Low Capacitance, Low Charge Injection, ±15 V/+12 V iCMOSTM Quad SPST Switches," Analog Devices Inc, 2005.
About the author
Marián Štofka is with Slovak University of Technology in Bratislava, Slovakia

This article is a Design Idea selected for re-publication by the editors. It was first published on July 15, 2010 in

 First Page Previous Page 1 • 2

Want to more of this to be delivered to you for FREE?

Subscribe to EDN Asia alerts and receive the latest design ideas and product news in your inbox.

Got to make sure you're not a robot. Please enter the code displayed on the right.

Time to activate your subscription - it's easy!

We have sent an activate request to your registerd e-email. Simply click on the link to activate your subscription.

We're doing this to protect your privacy and ensure you successfully receive your e-mail alerts.

Add New Comment
Visitor (To avoid code verification, simply login or register with us. It is fast and free!)
*Verify code:
Tech Impact

Regional Roundup
Control this smart glass with the blink of an eye
K-Glass 2 detects users' eye movements to point the cursor to recognise computer icons or objects in the Internet, and uses winks for commands. The researchers call this interface the "i-Mouse."

GlobalFoundries extends grants to Singapore students
ARM, Tencent Games team up to improve mobile gaming

News | Products | Design Features | Regional Roundup | Tech Impact