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AD7992 Datasheet(PDF) 11 Page - Analog Devices |
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AD7992 Datasheet(HTML) 11 Page - Analog Devices |
11 / 21 page AD7992 –11– REV. PrH PRELIMINARYTECHNICAL DATA Analog Input Figure 6 shows an equivalent circuit of the analog input sturcture of the AD7992. The two diodes D1 and D2 provide ESD protection for the analog inputs. Care must be taken to ensure that the analog input signal never ex- ceeds the supply rails by more than 300mV. This will cause these diodes to become forward biased and start conducting current into the substrate. 10 mA is the maxi- mum current these diodes can conduct without causing irreversable damage to the part. The capacitor C1 in Fig- ure 6 is typically about 4pF and can primarily be attrib- uted to pin capacitance. The resistor R1 is a lumped component made up of the on resistance (RON) of a switch (track and hold switch) and also includes the RON of the input multiplexer. This resistor is typically about 100 Ω. The capacitor C2 is the ADC sampling capacitor and has a capacitance of 30 pF typically. For ac applications, removing high frequency compo- nents from the analog input signal is recommended by use of an RC band-pass filter on the relevant analog input pin. In applications where harmonic distortion and signal to noise ratio are critical the analog input should be driven from a low impedance source. Large source impedances will significantly affect the ac performance of the ADC. This may necessitate the use of an input buffer amplifier. The choice of the op amp will be a function of the par- ticular application. When no amplifier is used to drive the analog input the source impedance should be limited to low values. The maximum source impedance will depend on the amount of total harmonic distortion (THD) that can be tolerated. The THD will increase as the source impedance increases and performance will degrade. Figure 7 shows a graph of the Total Harmonic Distortion vs. analog input signal frequency for different source impedances when using a supply voltage of 3V±10% and 5V ±10% and sampling at a rate of xkSPS. Figure 8 shows a graph of the total har- monic distortion versus analog input signal frequency for various supply voltages while sampling at xkSPS . Figure 6. Equivalent Analog Input Circuit Figure 7. THD vs. Analog Input Frequency for Various Source Impedance for VDD= 3V and 5V Figure 8. THD vs. Analog Input Frequency, Fs = xkSPS Figure 5 AD7992 Typical Connection Diagram VDD VINx GND +5V SUPPLY REF 19X 0.1µF 10µF AD7992 0V to REFIN INPUT SDA µC/µP SCL TWO WIRE SERIAL INTERFACE CONVST 0.1µF ALERT REF IN RP RP RP SET TO REQUIRED ADDRESS AS VIN D1 VDD D2 R1 C2 30PF C1 4PF CONVERSION PHASE - SWITCH OPEN TRACK PHASE - SWITCH CLOSED |
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