Re: OP Frage: DC Verstärkung unipolar nah e 0V

Joerg wrote:
Hallo Robert,


F?r die Experimentierschaltung hoff ich mal, dass einer der OP's den Offset sogar m”glichst <0 hat, damit ich vielleicht sogar negativen R?ckstrom von 0 unterscheiden kann... Oder gibts einen einfachen Trick, wie man das Sichtfenster definiert in den negativen Bereich erweitern kann?



Bau nen Spannungsteiler dazwischen:
Anstatt den Shunt direkt an (+) des Opv zu hängen, einfach nen Spannungsteiler zur Vref einfügen:

gnd - shunt - R1 - OPV(+) - R2 - Referenz


Stimmt, wenn man eine Vref hat, gehoert R2 da dran und nicht an V+.


Der AVR gibt die AREF aus - man darf sie aber nicht belasten. Müsste einen weiteren OPV freimachen.

Aber man kann es vielleicht mit dem anderen Trick und einen weiteren R kombinieren:
R2 gegen VCCA und einem abgestimmten Widerstand R3 von VCCA nach OPV(-):



+---+---------+----- VCCA
.-. .-. |
R2 | | | | R3 |
| | | | |
'-' '-' R5 |
| | ___ |
| +-|___|---|----+
| | | |
- OUT | | | |
| | | |\| |
| R1 | +-------|-\ |
| ___ | | | >---+----- ADC0
+--|___|-+-----------|+/
| | |/|
.-. .-. |
| | | | |
RMS 0.0xx | | | |R4 |
'-' '-' |
| | |
-IN -----------+------------+---------+-----------------------


Ist R3 zu groß, dann erscheint VCCA positiv am ADC0. Zu klein, dann negativ.
Es könnte einen R3 so geben, dass der Einfluss von VCCA Null wird...


Waere aber ziemlicher Krampf, wenn VCCA nicht sehr stabil ist. Kann man

Die Hypothese wäre ja eben, dass bei eingermaßen richtiger Wahl von R3 der Einfluss von VCCA (oder einen beliebigen hochgenugigen Spannung) "völlig" egalisiert wird. Aber so ganz sicher bin ich mir auch noch nicht.


VREF denn nicht wengistens mit den paar Mikroampere eines 1M Widerstandes belasten? Wenn nicht, warum hat Atmel sie dann ueberhaupt herausgefuehrt?


Dort soll man einen C ranmachen - und man kann sich ggf. dort stromlos vergleichen.


"ADC Voltage Reference: The reference voltage for the ADC (VREF) indicates the conversion range for the ADC.
Single ended channels that exceed VREF will result in codes close to 0x3FF. VREF can be
selected as either AVCC, internal 2.56V reference, or external AREF pin.
AVCC is connected to the ADC through a passive switch. The internal 2.56V reference is
generated from the internal bandgap reference (VBG) through an internal amplifier. In
either case, the external AREF pin is directly connected to the ADC, and the reference
voltage can be made more immune to noise by connecting a capacitor between the
AREF pin and ground. VREF can also be measured at the AREF pin with a high impedant
voltmeter. Note that VREF is a high impedant source, and only a capacitive load should
be connected in a system.
If the user has a fixed voltage source connected to the AREF pin, the user may not use
the other reference voltage options in the application, as they will be shorted to the
external voltage. If no external voltage is applied to the AREF pin, the user may switch
between AVCC and 2.56V as reference selection. The first ADC conversion result after
switching reference voltage source may be inaccurate, and the user is advised to discard
this result."



Grüsse
Robert


.

Relevant Pages

  • Re: Biasing ADC input
    ... Easiest and most accurate solution is using the "voltage divider" with two ... resistors connected to the reference. ... #2 would be the solution (if ADC doesn't sink enough, ... Take another 1k resistor and connect it between the reference ...
    (sci.electronics.basics)
  • Re: PIC based Pt100 RTD temp sensor circuit
    ... Something to amplify the small voltage across the RTD in to the range of an ADC. ... In combination with the opamp, it forces the voltage drop across the grounded 2.5K resistor to be a constant 2.5 volts, so the RTD current is forced to be a constant 1 mA. ... You may also want to reference the ADC full scale voltage to a precision reference, instead of using the 5 volt supply as the full scale value, or else the accuracy of the PIC supply gets involved in the measurement. ...
    (sci.electronics.basics)
  • Re: Need input on high voltage regulator design
    ... > What does the ADC use as a reference? ... which is provided by a 5-volt linear regulator in this ... can't select a different reference. ... With the voltage doubler stage after the secondary, ...
    (sci.electronics.design)
  • Re: differences between shunt and series voltage references?
    ... >> Hi - I need a 4.096 voltage reference for an ADC. ... From what I can tell a shunt V ... >> reference is just a zener diode of the rated voltage. ...
    (sci.electronics.design)
  • Re: Battery type for calibration reference
    ... My question has to do with how to provide a voltage calibration ... the best absolute voltage reference a non-technical person can buy in a ... Still, copper sulfate pentahydrate ... That's three metals ...
    (sci.electronics.basics)