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AD7984 датащи(PDF) 8 Page - Analog Devices |
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AD7984 датащи(HTML) 8 Page - Analog Devices |
8 / 12 page CN-0269 Circuit Note Rev. 0 | Page 8 of 12 Figure 13. AD8065 Noise Model The noise sources shown in Figure 13 must be converted to the output by multiplying the noise gain, which is 1 for a unity-gain buffer. eAD8065_RTO = 2 2 2 2 2 2 2 2 ) ( p f p S P V Rf RS RP i R i R R e e e e + + + + + + The noise from resistors can be calculated from the equation: Hz nV/ 1000 4 R e R × = at 25°C where R is in Ω. eRP = 2.2 nV/√Hz eRS =eRf= 4 nV/√Hz eV = 7 nV/√Hz ip = iN = 1 pA/√Hz eVAD8065 = 10 nV/√Hz The Noise of the AD8475Attenuation Stage The eAD8065_RTO term is the noise from the circuit at the input to AD8475 stage. This noise is reflected to the output of the AD8475 by multiplying the signal gain (0.4) of AD8475 stage as shown in Figure 14. Figure 14. AD8475 Noise Model The AD8475 output voltage noise is also 10 nV/√Hz, including amplifier voltage and current noise, as well as noise of internal resistors. The noise density of the whole signal chain in front of ADC is ( ) 2 2 _ 2 AD8475_RTO AD8475_RTO AD8475 GAIN TOTAL e e GAIN e + × × = For the ±10 V input range, the GAINAD8475 = 0.4. eTOTAL_0.4 = 11.5 nV/√Hz For the ±5 V input range, the GAINAD8475 = 0.8. eTOTAL_0.8 = 15.1 nV/√Hz The total output noise of the AD8475 is applied to the RC filter (10 Ω, 2.2 nF) that has a bandwidth of 7.23 MHz. The bandwidth of the AD8065 is 145 MHz, and the bandwidth of the AD8475 is 150 MHz. The input bandwidth of the AD7984 ADC is 10 MHz, therefore the noise at the input of the AD7984 is limited by the RC noise filter to 7.23 MHz. When the AD8475 is operating at a gain of 0.8 (worst case noise condition) the input rms noise to the ADC is therefore μV 51 MHz 23 . 7 57 . 1 Hz nV/ (15.1 _ = × × = RMS TOTAL V VTOTAL_PP = 6.6 × 51 µV = 337 µV For the 18-bit AD7984 with reference voltage of 4.096 V, the differential input span is 8.196 V. The LSB value is 31 µV. The peak-to-peak noise of 337 µV therefore corresponds to 11 LSBs peak-to-peak. Effect of Multiplexer Switching Transients The multiplexer has source and drain capacitance. The drain capacitance of the multiplexer holds the voltage of previous input channel. When the multiplexer switches to the next channel, this can create a transient or kick-back glitch through the RON resistance. This transient can affect the next conversion. Therefore, the pre-filter driver needs to have a very low output impedance and a fast settling time to the transient. Figure 15. Multiplexer Switching Transients The driver needs to be able to charge the input to the required accuracy (forward-charge) before the switch opens. The back- charge occurs when the switch opens, and generally is short and doesn’t present a problem. In order to make the circuit easy to drive, a buffer can be placed in front of the multiplexer (front buffer). The evaluation board EVAL-CN0269-SDPZ has footprints for the input buffer on each input channel and has an AD8065 installed in Channel 1 to Channel 4. Adding the buffer slightly increases the noise density and the settling time. However, in a practical application, the parasitic inductance and capacitance from the input cable or terminal connector will significantly increase the time of settling time and generate ringing due to the forward and back charge without the buffer. The additional input buffer isolates the parasitic effects and provides very low impedance to the multiplexer. The difference in performance between the circuit with or without input buffer is shown in the test part of this circuit note. Another reason for adding the input buffer is for that an additional filter can be placed ahead of it for anti-aliasing and noise reduction. AI RS RP AD8065 eRp eRs + – Rf eRf eV iP iN eAD8065_RTO eAD8475_RTO VN VP eAD8065_RTO AD8475 + – D CS CS CS CD CD VSS VSS VSS VSS SW1 SW2 RP VSS VSS VSS ADG5208 ADG5236 RON RON RON CP CP RP AI 1 AI 2 PRE-FILTER RS CIN +10V –10V P1 P2 P3 P1 P2 P3 SW_A0 KICK BACK-CHARGE FORWARD-CHARGE |
Аналогичный номер детали - AD7984 |
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Аналогичное описание - AD7984 |
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