Hallöchen,
habe da eine kleine Unklarheit was die Behandlung der Vref betrifft.
Der Wandler hat einen differenziellen Eingang.
Hier mal die nicht so leicht für mich verständlichen Teile aus dem
Datasheet:
1 | 3.1 Voltage Reference (VREF)
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2 | The MCP3550/1/3 devices accept single-ended reference
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3 | voltages from 0.1V to VDD. Since the converter
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4 | output noise is dominated by thermal noise, which is
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5 | independent of the reference voltage, the output noise
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6 | is not significantly improved by diminishing the reference
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7 | voltage at the VREF input pin. A reduced voltage
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8 | reference will significantly improve the INL performance
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9 | (see Figure 2-4); the INL max error is
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10 | proportional to VREF²
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1 | 4.5 Voltage Reference Input Pin
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2 | The MCP3550/1/3 devices accept a single-ended
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3 | external reference voltage, to be connected on the
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4 | VREF input pin. Internally, the reference voltage for the
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5 | ADC is a differential voltage with the non-inverting input
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6 | connected to the VREF pin and the inverting input
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7 | connected to the VSS pin. The value of the reference
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8 | voltage is VREF - VSS and the common mode of the
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9 | reference is always (VREF - VSS)/2.
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10 | The MCP3550/1/3 devices accept a single-ended
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11 | reference voltage from 0.1V to VDD. The converter
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12 | output noise is dominated by thermal noise that is
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13 | independent of the reference voltage. Therefore, the
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14 | output noise is not significantly improved by lowering
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15 | the reference voltage at the VREF input pin. However, a
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16 | reduced reference voltage will significantly improve the
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17 | INL performance since the INL max error is
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18 | proportional to VREF
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19 | 2 (see Figure 2-4).
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20 | The charge and discharge of the input capacitor create
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21 | dynamic input currents at the VREF input pin inversely
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22 | proportional to the sampling capacitor, which is a function
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23 | of the input reference voltage. The typical value of
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24 | the single-ended input impedance is 2.4 MΩ, with
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25 | VDD = VREF = 5V. The DC leakage current caused by
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26 | the ESD input diodes, though on the order of 1 nA
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27 | typically, can cause additional gain error proportional to
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28 | the source resistance at the VREF pin.
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Ich habe eine Vref mit 4,096V geplant.
welche Spannung kann er damit messen???
4Volt?
oder wegen dem (...the common mode of the
reference is always (VREF - VSS)/2.) nur 2,048?
Vielleicht kann mich jemand ja aufklären ;)
Habt Dank im vorraus
Anselm
p.S.: Vin- an GND/VSS angeschlossen
Vin+ am Ausgang des Messverstärkers
Datenblatt zu finden unter:
http://ww1.microchip.com/downloads/en/DeviceDoc/21950d.pdf