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AD7612 Datasheet(PDF) 21 Page - Analog Devices |
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AD7612 Datasheet(HTML) 21 Page - Analog Devices |
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21 / 32 page  Data Sheet AD7612 Rev. A | Page 21 of 32 DRIVER AMPLIFIER CHOICE Although the AD7612 is easy to drive, the driver amplifier must meet the following requirements: For multichannel, multiplexed applications, the driver amplifier and the AD7612 analog input circuit must be able to settle for a full-scale step of the capacitor array at a 16-bit level (0.0015%). For the amplifier, settling at 0.1% to 0.01% is more commonly specified. This differs signifi- cantly from the settling time at a 16-bit level and should be verified prior to driver selection. The AD8021 op amp combines ultra-low noise and high gain bandwidth and meets this settling time requirement even when used with gains of up to 13. The noise generated by the driver amplifier needs to be kept as low as possible to preserve the SNR and transition noise performance of the AD7612. The noise coming from the driver is filtered by the external 1-pole low-pass filter as shown in Figure 27. The SNR degradation due to the amplifier is 2 3 2 2 log 20 N dB NADC NADC LOSS Ne f V V SNR where: VNADC is the noise of the ADC, which is: 20 10 2 2 SNR INp-p NADC V V f–3dB is the cutoff frequency of the input filter (3.9 MHz). N is the noise factor of the amplifier (+1 in buffer configuration). eN is the equivalent input voltage noise density of the op amp, in nV/√Hz. The driver needs to have a THD performance suitable to that of the AD7612. Figure 15 shows the THD vs. frequency that the driver should exceed. The AD8021 meets these requirements and is appropriate for almost all applications. The AD8021 needs a 10 pF external compensation capacitor that should have good linearity as an NPO ceramic or mica type. Moreover, the use of a noninverting +1 gain arrangement is recommended and helps to obtain the best signal-to-noise ratio. The AD8022 can also be used when a dual version is needed and a gain of 1 is present. The AD829 is an alternative in appli- cations where high frequency (above 100 kHz) performance is not required. In applications with a gain of 1, an 82 pF compensation capacitor is required. The AD8610 is an option when low bias current is needed in low frequency applications. Since the AD7612 uses a large geometry, high voltage input switch, the best linearity performance is obtained when using the amplifier at its maximum full power bandwidth. Gaining the amplifier to make use of the more dynamic range of the ADC results in increased linearity errors. For applications requiring more resolution, the use of an additional amplifier with gain should precede a unity follower driving the AD7612. See Table 8 for a list of recommended op amps. Table 8. Recommended Driver Amplifiers Amplifier Typical Application ADA4841-x 12 V supply, very low noise, low distortion, low power, low frequency AD829 ±15 V supplies, very low noise, low frequency AD8021 ±12 V supplies, very low noise, high frequency AD8022 ±12 V supplies, very low noise, high frequency, dual AD8610/AD8620 ±13 V supplies, low bias current, low frequency, single/dual VOLTAGE REFERENCE INPUT/OUTPUT The AD7612 allows the choice of either a very low temperature drift internal voltage reference, an external reference or an external buffered reference. The internal reference of the AD7612 provides excellent perfor- mance and can be used in almost all applications. However, the linearity performance is guaranteed only with an external reference. |
Similar Part No. - AD7612_15 |
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Similar Description - AD7612_15 |
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