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AD7980 Datasheet(PDF) 12 Page - Analog Devices |
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AD7980 Datasheet(HTML) 12 Page - Analog Devices |
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12 / 22 page 
AD7980 Preliminary Technical Data Rev Pr C | Page 12 of 22 than 0.3 V because this causes these diodes to begin to forward- bias and start conducting current. These diodes can handle a forward-biased current of 130 mA maximum. For instance, these conditions could eventually occur when the input buffer’s (U1) supplies are different from VDD. In such a case, an input buffer with a short-circuit, current limitation can be used to protect the part. CIN RIN D1 D2 CPIN IN+ OR IN– GND VDD Figure 14. Equivalent Analog Input Circuit The analog input structure allows the sampling of the true differential signal between IN+ and IN−. By using these differential inputs, signals common to both inputs are rejected. During the acquisition phase, the impedance of the analog inputs (IN+ or IN−) can be modeled as a parallel combination of capacitor, CPIN, and the network formed by the series connection of RIN and CIN. CPIN is primarily the pin capacitance. RIN is typically 400 Ω and is a lumped component made up of some serial resistors and the on resistance of the switches. CIN is typically 30 pF and is mainly the ADC sampling capacitor. During the conversion phase, where the switches are opened, the input impedance is limited to CPIN. RIN and CIN make a 1- pole, low-pass filter that reduces undesirable aliasing effects and limits the noise. When the source impedance of the driving circuit is low, the AD7980 can be driven directly. Large source impedances significantly affect the ac performance, especially total harmonic distortion (THD). The dc performances are less sensitive to the input impedance. The maximum source impedance depends on the amount of THD that can be tolerated. The THD degrades as a function of the source impedance and the maximum input frequency. DRIVER AMPLIFIER CHOICE Although the AD7980 is easy to drive, the driver amplifier needs to meet the following requirements: • The noise generated by the driver amplifier needs to be kept as low as possible in order to preserve the SNR and transition noise performance of the AD7980. The noise coming from the driver is filtered by the AD7980 analog input circuit 1-pole, low-pass filter made by RIN and CIN or by the external filter, if one is used. Because the typical noise of the AD7980 is 38 μV rms, the SNR degradation due to the amplifier is ⎟ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎜ ⎝ ⎛ + = − 2 2 ) ( 2 π 44 44 20log N 3dB LOSS Ne f SNR where: f–3dB is the input bandwidth in MHz of the AD7980 (10MHz) or the cutoff frequency of the input filter, if one is used. N is the noise gain of the amplifier (for example, +1 in buffer configuration). eN is the equivalent input noise voltage of the op amp, in nV/√Hz. • For ac applications, the driver should have a THD performance commensurate with the AD7980. • For multichannel multiplexed applications, the driver amplifier and the AD7980 analog input circuit must settle for a full-scale step onto the capacitor array at a 16-bit level (0.0015%, 15 ppm). In the amplifier’s data sheet, settling at 0.1% to 0.01% is more commonly specified. This could differ significantly from the settling time at a 16-bit level and should be verified prior to driver selection. Table 8. Recommended Driver Amplifiers Amplifier Typical Application ADA4941 Very low noise, low power single to Differential ADA4841 Very low noise, small and low power AD8021 Very low noise and high frequency AD8022 Low noise and high frequency OP184 Low power, low noise, and low frequency AD8655 5 V single-supply, low noise AD8605, AD8615 5 V single-supply, low power |
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