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AD7828LP Datasheet(PDF) 7 Page - Analog Devices |
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AD7828LP Datasheet(HTML) 7 Page - Analog Devices |
7 / 16 page AD7824/AD7828 REV. F –7– INPUT CURRENT Due to the novel conversion techniques employed by the AD7824/ AD7828, the analog input behaves somewhat differently than in conventional devices. The ADC’s sampled-data comparators take varying amounts of input current depending on which cycle the conversion is in. The equivalent input circuit of the AD7824/AD7828 is shown in Figure 8. When a conversion starts ( CS and RD going low), all input switches close, and the selected input channel is con- nected to the most significant and least significant comparators. Therefore, the analog input is simultaneously connected to 31 input capacitors of 1 pF each. 1pF 1pF • • • 15LSB COMPARATORS 1pF 1pF • • • 16MSB COMPARATORS TO LS LADDER RON RON R MUX CS 12pF CS 2pF RS AIN1 TO MS LADDER VIN AD7824/ AD7828 Figure 8. AD7824/AD7828 Equivalent Input Circuit The input capacitors must charge to the input voltage through the on resistance of the analog switches (about 3 k Ω to 6 kΩ). In addition, about 14 pF of input stray capacitance must be charged. The analog input for any channel can be modelled as an RC network, as shown in Figure 9. As RS increases, it takes longer for the input capacitance to charge. RON 350 R MUX 800 CS1 12pF RS VIN CS2 2pF 31pF AIN1 Figure 9. RC Network Model The time for which the input comparators track the analog input is approximately 1 µs at the start of conversion. Because of input transients on the analog inputs, it is recommended that a source impedance no greater than 100 Ω be connected to the analog inputs. The output impedance of an op amp is equal to the open loop output impedance divided by the loop gain at the frequency of interest. It is important that the amplifier driving the AD7824/ AD7828 analog inputs have sufficient loop gain at the input signal frequency as to make the output impedance low. Suitable op amps for driving the AD7824/AD7828 are the AD544 or AD644. INHERENT SAMPLE-HOLD A major benefit of the AD7824’s and AD7828’s analog input structure is its ability to measure a variety of high speed signals without the help of an external sample-and-hold. In a conven- tional SAR type converter, regardless of its speed, the input must remain stable to at least 1/2 LSB throughout the conversion process if rated accuracy is to be maintained. Consequently, for many high speed signals, this signal must be externally sampled and held stationary during the conversion. The AD7824/AD7828 input comparators, by nature of their input switching, inherently accomplish this sample-and-hold function. Although the conver- sion time for AD7824/AD7828 is 2 µs, the time for which any selected analog input must be 1/2 LSB stable is much smaller. The AD7824/AD7828 tracks the selected input channel for approximately 1 µs after conversion start. The value of the analog input at that instant (1 µs from conversion start) is the measured value. This value is then used in the least significant flash to generate the lower four bits of data. SINUSOIDAL INPUTS The AD7824/AD7828 can measure input signals with slew rates as high as 157 mV/ µs to the rated specifications. This means that the analog input frequency can be up to 10 kHz without the aid of an external sample-and-hold. Furthermore, the AD7828 can measure eight 10 kHz signals without a sample-and-hold. The Nyquist criterion requires that the sampling rate be twice the input frequency (i.e., 2 × 10 kHz). This requires an ideal anti- aliasing filter with an infinite roll-off. To ease the problem of antialiasing filter design, the sampling rate is usually much greater than the Nyquist criterion. The maximum sampling rate (FMAX) for the AD7824/AD7828 can be calculated as follows: F tt MAX CRD P = + 1 F EE kHz MAX = + = 1 26 0 5 6 400 –. – tCRD = AD7824/AD7828 Conversion Time tP = Minimum Delay Between Conversion This permits a maximum sampling rate of 50 kHz for each of the eight channels when using the AD7828 and 100 kHz for each of the four channels when using the AD7824. |
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