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HadCon 2 Fpga Project 1 WADC

Introduction

  • One-Wire ADC is an application implemented inside the FPGA of the HadCon2 board. It can provide 6 ADC channels of 10bit resolution at 9,6kSps that can be read through 1-wire interface from a Master device. The Span of the ADC is 150mV – 3,097V. Multiple versions of higher resolution can also be supported.
  • It is a compact design since it uses only the FPGA and an external Low pass filter. There is no need for additional ICs for the protocol and the ADCs.

Documentation

Layout

  • The connectors JMXOIO1 and JMXOIO2 of the HadCon2 board are used for all the external connections as shown at the following schematic. Connectors_(1).jpg

Design Block Diagram

  • Block Diagram:
    Block_Diagram

PWM and Low-Pass Filter

The idea

The idea of this ADC is to use a PWM signal from the FPGA and drive it to an external Low Pass Filter. From the output of this filter we will get a ramp waveform as reference voltage. This reference voltage and the unknown signal that we want to measure will be driven to the input pins of the LVDS input of the FPGA. Whenever the unknown signal crosses the reference voltage we will have a change to the output of the LVDS discriminator. With this way we can measure the unknown signal by knowing when exactly is at the same voltage of the known reference voltage.

PWM & Filter Diagram

PWM

To produce the ramp reference signal a PWM waveform with 100MHz clock frequency must be used. The PWM is changing its duty cycle step by step in order to produce a different average voltage after filtering. During the half period, the duty cycle is rising and the other half falling. Every side of the ramp waveform is made of 101 steps of different duty cycle (101^2)*(1/100MHz)=102us. To have a good PWM signal with less switching noise each step is made of a 101bit shift register that has distributed ‘1’s and ‘0’s in order to have as much small gaps between 1 and 0, as you can see below.

51% Duty Cycle: "111010101010101010101……01010101010101010101010101010101010"
50% Duty Cycle: "101010101010101010101……01010101010101010101010101010101010"
49% Duty Cycle: "001010101010101010101……01010101010101010101010101010101010"

Low-pass Filter

When the PWM signal passes from the LPF the high frequencies of the PWM are cutoff and at the output we have a clear ramp waveform. The filter that is used is a second order LC filter with cutoff frequency FC=33,9KHz as you can see at the schematic below. Also a ferrite ring with 40 turns is used to the output of the filter to attenuate the switching noise. To produce a waveform that seems like a ramp some of the first harmonics of its base frequency must be passed from the filter. In our case the ramp frequency is Framp=4,8KHz and we are passing 7 harmonics F7th=33,6KHz as you can see at the bode diagram.

  • Filter schematic:
    LC_filter_schematic.jpg

  • Bode diagram:
    Bode_diagram.jpg

  • Ramp waveform measurement:
    ramp_waveform.jpg

FPGA Design

TDC

The TDC of the system is made of a simple counter. When the ramp reference voltage is at the bottom or top edges the counter starts counting up or down. Whenever the discriminator’s output changes, the counter stops counting and saves this value. That value represents the voltage of the unknown signal by the following equation: 1LSB(3.025mV)* Value

  • TDC waveforms:
    TDC.jpg

Decoder

The decoder subcomponent is used to decode the Function Commands that have been received from the 1-wire interface and enable the correct channel of the ADC and transmit its value back to 1-wire. The Function Commands that this device supports are the following:

FUNCTION COMMAND HEX VALUE
Read CH.1 LSB byte 01
Read CH.1 MSB byte 41
Read CH.2 LSB byte 02
Read CH.2 MSB byte 42
Read CH.3 LSB byte 03
Read CH.3 MSB byte 43
Read CH.4 LSB byte 04
Read CH.4 MSB byte 44
Read CH.5 LSB byte 05
Read CH.5 MSB byte 45
Read CH.6 LSB byte 06
Read CH.6 MSB byte 46

One-Wire Interface

This ADC is used as a Slave device of the 1-Wire interface. The 1-wire interface port of the FPGA is a bidirectional open drain pin. The data are send in packets of 8bit with the LSB transmitted first. It supports all Maxim 1-wire Master devices at the standard speed of the 1-wire protocol (15,4kbps) and the data transmission is separated into 4 steps.
A) Reset-presence
B) ROM command
C) Function command
D) Data transmission

The timing slots of the supported 1-Wire protocol are described at the following graphs.

  • Reset Presence:
    Reset_-_Presence.jpg

  • Read Write:
    Read__Write.jpg

Documents & Presentations

-- PeterZumbruch - 30 Jan 2014
-- MichailPligouroudis - 16 Feb 2014
Topic attachments
I Attachment Action Size Date Who Comment
1-Wire_ADC_REPORT.pdfpdf 1-Wire_ADC_REPORT.pdf manage 1492.5 K 2014-02-17 - 22:56 MichailPligouroudis 1-Wire ADC report
BLOCK_DIAGRAM.jpgjpg BLOCK_DIAGRAM.jpg manage 59.4 K 2014-01-30 - 10:50 MichailPligouroudis block diagram
Bode_diagram.jpgjpg Bode_diagram.jpg manage 212.8 K 2014-02-19 - 20:35 MichailPligouroudis Bode diagram
Connectors_(1).jpgjpg Connectors_(1).jpg manage 86.2 K 2014-02-15 - 18:36 MichailPligouroudis connector schematic
LC_filter_schematic.jpgjpg LC_filter_schematic.jpg manage 14.8 K 2014-02-19 - 20:34 MichailPligouroudis LC filter schematic
PWM_filter_diagram.jpgjpg PWM_filter_diagram.jpg manage 38.6 K 2014-01-30 - 11:15 MichailPligouroudis PWM & Filter Diagram
Read__Write.jpgjpg Read__Write.jpg manage 232.7 K 2014-02-25 - 16:27 MichailPligouroudis Read Write
Reset_-_Presence.jpgjpg Reset_-_Presence.jpg manage 139.4 K 2014-02-25 - 16:26 MichailPligouroudis Reset Presence
TDC.jpgjpg TDC.jpg manage 39.6 K 2014-02-25 - 14:56 MichailPligouroudis TDC waveforms
ramp_waveform.jpgjpg ramp_waveform.jpg manage 47.2 K 2014-02-20 - 12:46 MichailPligouroudis Ramp Waveform
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Topic revision: r11 - 2014-02-25, MichailPligouroudis
 
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