adc
I have found this circuit. It's a RFID tag reader. It uses 2 op-amp to make the analogic to digital conversion.
I don't understand how does it works.
Can anyone explain how does it make the A/D conversion please?
Link to the circuit: http://www.serasidis.gr/circuits/RFID_reader/images/125kHz_RFID_reader_schem.GIF
That 1M is there just to make sure that the ADC input pin isn't totally free-floating, and also gives a known voltage (+5) when no switches are pressed. This keeps the ADC from picking up ambient noise from the rest of the circuit.
The reason for the value of 1M is a compromise between a value low enough to keep the ADC input from acting like an antenna, and a value high enough to not overly skew the result you get when a switch is pressed. When a switch is pressed, that 1M is in parallel with the 'top half' of a voltage divider formed by the 1K resistors either side of the switch.
Yes, you would need to use the Configuration Register to activate the internal reference. You would need to perform two 16-bit writes to the device using your parallel bus.
\$\overline{HW}/SW = 1\$ (tied high)
\$\overline{PAR}/SER = 0\$ (tied low)
First
\$\overline{CS} = 0\$
\$\overline{WR} = 0\$
\$DB[15:0] = ??????????????XX\$
then
\$\overline{CS} = 0\$
\$\overline{WR} = 1\$
\$DB[15:0] = XXXXXXXXXXXXXXXX\$
then
\$\overline{CS} = 0\$
\$\overline{WR} = 0\$
\$DB[15:0] = 10?XXX??????????\$
Where X is a don't care and ? means you will need to decide what is correct based on your application.
From the datasheet:
See "Software Mode" on Page 29 and "Configuration (CONFIG) Register" on Page 31 for more information.
Best Answer
It is not an A/D converter - it is just an amplifier.
To transmit to the card T1 is turned on and off by the microprocessor to create a 125kHz signal at R1, the antenna (L1) resonates with C2 to create a signal that is picked up by the card.
The card will detect this signal and modulate it by modifying the resonant antenna within the card, as it does so it will affect the amplitude of the 125 kHz signal at L1.
Diode D1 detects the 125kHz signal across the antenna, as the intensity of the signal changes so does the voltage across R5 fluctuate.
This small signal is amplified by IC2A by about 12 times. It is then amplified again by T2, the way that T2 is biased will cause it to limit the signal so only the negative peaks at IC2a pin 1 will cause the transistor to come out of saturation.
The output of T2 then passes through IC2B, by now the signal will be either high or low and so will be a digital signal that is sensed by the microprocessor to read the card data.