Circuit allows testing of sampleandhold amps
07 Aug 2015  Marian StofkaShare 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: V_{INJ}=6[ΔV_{OUT}]–2/3[Δ]–16/3V_{STAT}, and V_{DROPPEAK}=16[–[ΔV_{OUT}]+1/3[Δ]+2/3V_{STAT}]. You must find the optimal repetition rate, f_{REP}, of the logiccontrol 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: (–V_{STAT})/(V_{OUT}–V_{STAT})≈3. The following equation finds the maximum value for the optimal repetition rate: f_{REP}≤(0.01/4)×1/(t_{ON}–t_{OFF}), where t_{ON} and t_{OFF} are the on and off times, respectively. This equation ensures that the difference in values between the turnon and turnoff times of the sampleandhold 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 highperformance 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 lowvalue repetition rates. In this case, the repetition rate can be the value of the frequency at which –V_{STAT}≤1/10×V_{INMAX}, where V_{INMAX} is the maximum inputvoltage range. The best way to determine the lower limit of the repetition rate is through experimentation.
Figure 2: A sampleandhold 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 sampleandhold amplifier using the circuit in figure 2 uses a supply voltage of –1V, a draintodrain 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% dutycycle values for the AGD1213's internal switch control used input voltages of 0 and 2.5V. You will measure the outputvoltage 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, Q_{INJ}, into the hold capacitor, C_{H}=2 nF, is Q_{INJ}=C_{H}×V_{INJ}. 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: ΔQ_{INJ}=Q_{INJ}(2.5V)–Q_{INJ}(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: I_{LEAK}=C_{H}×V_{DROPPEAK}×f_{REP}, where I_{LEAK} 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.
Reference
"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 republication by the editors. It was first published on July 15, 2010 in EDN.com.
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.
Please enter the valid code. Sorry, you have reached the maximum number of requests allowed. You may wish to try again after a few hours.Time to activate your subscription  it's easy!
We have sent an activate request to your registerd eemail. Simply click on the link to activate your subscription.
We're doing this to protect your privacy and ensure you successfully receive your email alerts.

Robotic glove helps restore hand movements
The device is an improvement from conventional robotic hand rehabilitation devices as it has sensors to detect muscle signals and conforms to the natural movements of the human hand.
KGlass 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 "iMouse."