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AD585J Fiches technique(PDF) 5 Page - Analog Devices |
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AD585J Fiches technique(HTML) 5 Page - Analog Devices |
5 / 6 page AD585 REV. A –5– For the AD585 in particular it becomes: S/H Offset (V ) = 0.3 pC 100 pF + C EXT () The addition of an external hold capacitor also affects the acqui- sition time of the AD585. The change in acquisition time with respect to the CEXT is shown graphically in Figure 2. HOLD MODE In the hold mode there are two important specifications that must be considered; feedthrough and the droop rate. Feedthrough errors appear as an attenuated version of the input at the output while in the hold mode. Hold-Mode feedthrough varies with fre- quency, increasing at higher frequencies. Feedthrough is an im- portant specification when a sample and hold follows an analog multiplexer that switches among many different channels. Hold-mode droop rate is the change in output voltage per unit of time while in the hold mode. Hold-mode droop originates as leakage from the hold capacitor, of which the major leakage current contributors are switch leakage current and bias current. The rate of voltage change on the capacitor dV/dT is the ratio of the total leakage current IL to the hold capacitance CH. Droop Rate = dVOUT dT (Volts/Sec) = IL ( pA) CH ( pF ) For the AD585 in particular; Droop Rate = 100 pA 100 pF + (C EXT ) Additionally the leakage current doubles for every 10 °C increase in temperature above 25 °C; therefore, the hold-mode droop rate characteristic will also double in the same fashion. The hold-mode droop rate can be traded-off with acquisition time to provide the best combination of droop error and acquisition time. The tradeoff is easily accomplished by varying the value of CEXT. Since a sample and hold is used typically in combination with an A/D converter, then the total droop in the output voltage has to be less than 1/2 LSB during the period of a conversion. The maximum allowable signal change on the input of an A/D converter is: ∆V max = Full -Scale Voltage 2 N +1 () Once the maximum ∆V is determined then the conversion time of the A/D converter (TCONV) is required to calculate the maxi- mum allowable dV/dT. dV dt max = ∆V max TCONV The maximum dV max dT as shown by the previous equation is the limit not only at 25 °C but at the maximum expected operat- ing temperature range. Therefore, over the operating temperature range the following criteria must be met (TOPERATION –25°C) = ∆T. dV 25 °C dT × 2 ∆T °C () 10 °C ≤ dV max dT HOLD-TO-SAMPLE TRANSITION The Nyquist theorem states that a band-limited signal which is sampled at a rate at least twice the maximum signal frequency can be reconstructed without loss of information. This means that a sampled data system must sample, convert and acquire the next point at a rate at least twice the signal frequency. Thus the maximum input frequency is equal to f MAX = 1 2 T ACQ + TCONV + TAP () Where TACQ is the acquisition time of the sample-to-hold amplifier, TAP is the maximum aperture time (small enough to be ignored) and TCONV is the conversion time of the A/D converter. DATA ACQUISITION SYSTEMS The fast acquisition time of the AD585 when used with a high speed A/D converter allows accurate digitization of high fre- quency signals and high throughput rates in multichannel data acquisition systems. The AD585 can be used with a number of different A/D converters to achieve high throughput rates. Fig- ures 12 and 13 show the use of an AD585 with the AD578 and AD574A. Figure 12. A/D Conversion System, 117.6 kHz Throughput 58.8 kHz max Signal Input Figure 13. 12-Bit A/D Conversion System, 26.3 kHz Throughput Rate, 13.1 kHz max Signal Input |
Numéro de pièce similaire - AD585J |
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Description similaire - AD585J |
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