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AD7674 Datasheet(PDF) 19 Page - Analog Devices |
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AD7674 Datasheet(HTML) 19 Page - Analog Devices |
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19 / 28 page  AD7674 Rev. 0 | Page 19 of 28 U2 8.25k Ω 2.5V AD8021 590 Ω AD7674 IN+ IN– REF U1 ANALOG INPUT (UNIPOLAR 0V TO 4.096V) 10pF AD8021 590 Ω 10pF 10 µF 100nF 1.82k Ω 2.7nF 15 Ω 2.7nF 15 Ω REFBUFIN 03083-0-031 The SNR degradation due to the amplifier is = π + 2 ) ( 625 25 log 20 N 3dB – LOSS Ne f SNR where: f–3dB is the –3 dB input bandwidth in MHz of the AD7674 (26 MHz) or the cutoff frequency of the input filter, if used. N is the noise factor of the amplifiers (1 if in buffer configuration). eN is the equivalent input noise voltage of each op amp in nV/√Hz. Figure 31. Single-Ended-to-Differential Driver Circuit (Internal Reference Buffer Used) Voltage Reference For instance, for a driver with an equivalent input noise of 2 nV/√Hz (e.g., AD8021) configured as a buffer, thus with a noise gain of +1, the SNR degrades by only 0.34 dB with the filter in Figure 27, and by 1.8 dB without it. The AD7674 allows the use of an external voltage reference either with or without the internal reference buffer. Using the internal reference buffer is recommended when sharing a common reference voltage between multiple ADCs is desired. • The driver needs to have a THD performance suitable to that of the AD7674. However, the advantages of using the external reference voltage directly are: The AD8021 meets these requirements and is usually appropriate for almost all applications. The AD8021 needs a 10 pF external compensation capacitor, which should have good linearity as an NPO ceramic or mica type. • • The SNR and dynamic range improvement (about 1.7 dB) resulting from the use of a reference voltage very close to the supply (5 V) instead of a typical 4.096 V reference when the internal buffer is used The AD8022 could be used if a dual version is needed and gain of 1 is present. The AD829 is an alternative in applications where high frequency (above 100 kHz) performance is not required. In gain of 1 applications, it requires an 82 pF compensation capacitor. The AD8610 is another option when low bias current is needed in low frequency applications. The power saving when the internal reference buffer is powered down (PDBUF High) To use the internal reference buffer, PDBUF should be LOW. A 2.5 V reference voltage applied on the REFBUFIN input will result in a 4.096 V reference on the REF pin. Single-to-Differential Driver For applications using unipolar analog signals, a single-ended- to-differential driver will allow for a differential input into the part. The schematic is shown in Figure 31. When provided an input signal of 0 to VREF, this configuration will produce a differential ±VREF with midscale at VREF/2. In both cases, the voltage reference input REF has a dynamic input impedance and therefore requires an efficient decoupling between REF and REFGND inputs, The decoupling consists of a low ESR 47 µF tantalum capacitor connected to the REF and REFGND inputs with minimum parasitic inductance. If the application can tolerate more noise, the AD8138 differential driver can be used. Care should also be taken with the reference temperature coefficient of the voltage reference, which directly affects the full-scale accuracy if this parameter matters. For instance, a ±4 ppm/°C temperature coefficient of the reference changes the full scale by ±1 LSB/°C. |
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