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AD7674 Datasheet(PDF) 18 Page - Analog Devices |
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AD7674 Datasheet(HTML) 18 Page - Analog Devices |
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18 / 28 page  AD7674 TYPICAL CONNECTION DIAGRAM Figure 27 shows a typical connection diagram for the AD7674. Different circuitry shown on this diagram is optional and is discussed later in this data sheet. Analog Inputs Figure 28 shows a simplified analog input section of the AD7674. The diodes shown in Figure 28 provide ESD protection for the inputs. Care must be taken to ensure that the analog input signal never exceeds the absolute ratings on these inputs. This will cause these diodes to become forward biased and start conducting current. These diodes can handle a forward-biased current of 120 mA max. This condition could eventually occur when the input buffer’s U1 or U2 supplies are different from AVDD. In such a case, an input buffer with a short-circuit current limitation can be used to protect the part. IN+ IN– AGND AVDD R+ = 102 Ω CS CS R– = 102 Ω 03083-0-028 Figure 28. Simplified Analog Input This analog input structure is a true differential structure. By using these differential inputs, signals common to both inputs are rejected as shown in Figure 29, which represents typical CMRR over frequency. FREQUECY (kHz) 66 64 50 100 1000 10000 110 62 60 58 56 54 52 03083-0-029 Figure 29. Analog Input CMRR vs. Frequency During the acquisition phase for ac signals, the AD7674 behaves like a 1-pole RC filter consisting of the equivalent resistance R+, R–, and CS. The resistors R+ and R– are typically 102 Ω and are lumped components made up of a serial resistor and the on resistance of the switches. CS is typically 60 pF and mainly consists of the ADC sampling capacitor. This 1-pole filter with a –3 dB cutoff frequency of 26 MHz typ reduces any undesirable aliasing effect and limits the noise coming from the inputs. Because the input impedance of the AD7674 is very high, the part can be driven directly by a low impedance source without gain error. This allows the user to put an external 1-pole RC filter between the amplifier output and the ADC analog inputs, as shown in Figure 27, to improve the noise filtering done by the AD7674 analog input circuit. However, the source impedance has to be kept low because it affects the ac performance, especially the total harmonic distortion (THD). The maximum source impedance depends on the amount of THD that can be tolerated. The THD degrades as a function of source impedance and the maximum input frequency, as shown in Figure 30. INPUT RESISTANCE ( Ω) –95 –120 45 75 105 15 –100 –105 –110 –115 20kHz 10kHz 2kHz 03083-0-030 Figure 30. THD vs. Analog Input Frequency and Source Resistance Driver Amplifier Choice Although the AD7674 is easy to drive, the driver amplifier needs to meet the following requirements: • • The driver amplifier and the AD7674 analog input circuit have to be able to settle for a full-scale step of the capacitor array at an 18-bit level (0.0004%). In the amplifier’s data sheet, settling at 0.1% or 0.01% is more commonly specified. This could differ significantly from the settling time at an 18-bit level and, therefore, should be verified prior to driver selection. The tiny op amp AD8021, which combines ultralow noise and high gain-bandwidth, meets this settling time requirement. 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 AD7674. The noise coming from the driver is filtered by the AD7674 analog input circuit 1-pole low-pass filter made by R+, R–, and CS. Rev. 0 | Page 18 of 28 |
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