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EDNE JAN 2017

Analog Tips A termination resistor in parallel with the input can be used if required. The ADA4940-1internal common-mode feedback loop forces common-mode output voltage to equal the voltage applied to the VOCM input and offers an excellent output balance. The differential output voltage depends on VOCM when two feedback factors β1 and β2 are not equal and any imbalance in output amplitude or phase produces an undesirable commonmode component in the output and causes a redundant noise and offset in the differential output. Therefore, it’s imperative that the combination of input source impedance and R1 (R3) should be 1 kΩ in this case (that is, β1 = β2) to avoid the mismatch in the commonmode voltage of each output signal and prevent the increase in common-mode noise coming from the ADA4940-1. As signals travel through the traces of a printed-circuit board (PCB) and long cables, system noise accumulates in the signals and a differential input ADC rejects any signal noise that appears as a common-mode voltage. The expected signal-to-noise ratio (SNR) of this 18-bit, 1 MSPS data acquisition system can be calculated theoretically by taking the root sum square (RSS) of each noise source – ADA4940- 1, ADR435 and AD7982. The ADA4940-1offers low noise performance of typically 3.9 nV/√Hz at 100 kHz as shown in Figure 2. Figure 2 ADA4940 input voltage noise spectral density vs. frequency. It is important to calculate the noise gain of the differential amplifier in order to find its equivalent output noise contribution. The noise gain of the differential amplifier is: where; and are two feedback factors. The following differential amplifier noise sources should be taken into account: Since the ADA4940-1 input voltage noise is 3.9 nV/√Hz, its differential output noise would be 7.8 nV/√Hz. The ADA4940-1’s common-mode input voltage noise (eOCM) is 83 nV/√Hz from the data sheet, so its output noise would be; – eOCM × (β1 – β2) × NG = 0 Noise from the R1, R2, R3, and R4 resistors can be calculated based on the Johnson- Nyquist noise equation for a given bandwidth. eRn= √(4kB TR) where kB is the Boltzmann constant, (1.38065 × 10-23 J/K), T is the resistor’s absolute temperature in Kelvin, and R is the resistor value in ohms (Ω). 28 EDN Europe January 2017 www.edn-europe.com


EDNE JAN 2017
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