Code for reading voltages with the Precision Voltage Shield, an Arduino shield for reading very precisely. 8 channels; 14-bit, 16-bit, and 18-bit versions. More details at http://rascalmicro.com/blog/2013/03/21/bringing-an-analog-voltage-arduino-shield-to-life/

precision_voltage_shield.ino

// Copyright 2014, Brandon Stafford, brandon@rascalmicro.com

// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions
// are met:

// 1. Redistributions of source code must retain the above copyright
//    notice, this list of conditions and the following disclaimer.
// 2. Redistributions in binary form must reproduce the above copyright
//    notice, this list of conditions and the following disclaimer in the
//    documentation and/or other materials provided with the distribution.
// 3. The name of the author may not be used to endorse or promote products
//    derived from this software without specific prior written permission.

// THIS SOFTWARE IS PROVIDED BY THE AUTHOR "AS IS" AND ANY EXPRESS OR
// IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
// OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
// IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
// INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
// NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
// THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

#include <SPI.h>

#define RESOLUTION 16

#if RESOLUTION == 14
#define SCALE_FACTOR 0.0006103515625
#endif

#if RESOLUTION == 16
#define SCALE_FACTOR 0.000152587890625
#endif

#if RESOLUTION == 18
#define SCALE_FACTOR 0.00003814697265625
#endif

// 10/(2^14) = 0.0006103515625
// 10/(2^16) = 0.000152587890625
// 10/(2^18) = 0.00003814697265625

#define BUSY 3
#define RESET 4
#define START_CONVERSION 5
#define CHIP_SELECT 10
#define MISO 12
#define LED 13
#define TOTAL_RAW_BYTES RESOLUTION

int bytesToRead = TOTAL_RAW_BYTES;
byte raw[TOTAL_RAW_BYTES];
signed long parsed[8];

void setup() {
  pinMode(BUSY, INPUT);
  pinMode(RESET, OUTPUT);
  pinMode(LED, OUTPUT);
  pinMode(START_CONVERSION, OUTPUT);
  pinMode(MISO, INPUT);

  Serial.begin(115200);
  SPI.begin();
  
  digitalWrite(START_CONVERSION, HIGH);  
  digitalWrite(CHIP_SELECT, HIGH);
  digitalWrite(RESET, HIGH);
  delay(1);
  digitalWrite(RESET, LOW);
}

void loop() {
  int i;

  digitalWrite(START_CONVERSION, LOW);
  delayMicroseconds(10);
  digitalWrite(START_CONVERSION, HIGH);

  while (digitalRead(BUSY) == HIGH) {
    // wait for conversion to complete
  }
  digitalWrite(CHIP_SELECT, LOW);
  while (bytesToRead > 0) {
    raw[TOTAL_RAW_BYTES - bytesToRead] = SPI.transfer(0x00);
    bytesToRead--;
  }
  digitalWrite(CHIP_SELECT, HIGH);
  bytesToRead = TOTAL_RAW_BYTES;
  
  parseRawBytes();

  //Serial.write(raw, 16);

  for(i=0; i<8; i++) {
    Serial.print((float)parsed[i] * SCALE_FACTOR, 5);
    Serial.print(",");
  }
  Serial.print("\r\n");
  delay(1000);
}

void parseRawBytes() {
  int i;
  
#if RESOLUTION == 14
      parsed[0] = (raw[0] << 6) + (raw[1] >> 2) & 0x3FFF;
      parsed[1] = (raw[1] << 12) + (raw[2] << 4) + (raw[3] >> 4) & 0x3FFF;
      parsed[2] = (raw[3] << 10) + (raw[4] << 2) + (raw[5] >> 6) & 0x3FFF;
      parsed[3] = (raw[5] << 8) + (raw[6] << 0) & 0x3FFF;
      parsed[4] = (raw[7] << 6) + (raw[8] >> 2 ) & 0x3FFF;
      parsed[5] = (raw[8] << 12) + (raw[9] << 4) + (raw[10] >> 4) & 0x3FFF;
      parsed[6] = (raw[10] << 10) + (raw[11] << 2) + (raw[12] >> 6) & 0x3FFF;
      parsed[7] = (raw[12] << 8) + (raw[13] << 0) & 0x3FFF;
#endif

#if RESOLUTION == 16 // 16-bit code not tested yet
      parsed[0] = (raw[0] << 8) + (raw[1] >> 0);
      parsed[1] = (raw[2] << 8) + (raw[3] >> 0);
      parsed[2] = (raw[4] << 8) + (raw[5] >> 0);
      parsed[3] = (raw[6] << 8) + (raw[7] >> 0);
      parsed[4] = (raw[8] << 8) + (raw[9] >> 0);
      parsed[5] = (raw[10] << 8) + (raw[11] >> 0);
      parsed[6] = (raw[12] << 8) + (raw[13] >> 0);
      parsed[7] = (raw[14] << 8) + (raw[15] >> 0);
#endif

#if RESOLUTION == 18
      parsed[0] = (raw[0] << 10) + (raw[1] << 2) + (raw[2] >> 6) & 0x3FFFF;
      parsed[1] = (raw[2] << 12) + (raw[3] << 4) + (raw[4] >> 4) & 0x3FFFF;
      parsed[2] = (raw[4] << 14) + (raw[5] << 6) + (raw[6] >> 2) & 0x3FFFF;
      parsed[3] = (raw[6] << 16) + (raw[7] << 8) + (raw[8] >> 0) & 0x3FFFF;
      parsed[4] = (raw[9] << 10) + (raw[10] << 2) + (raw[11] >> 6) & 0x3FFFF;
      parsed[5] = (raw[11] << 12) + (raw[12] << 4) + (raw[13] >> 4) & 0x3FFFF;
      parsed[6] = (raw[13] << 14) + (raw[14] << 6) + (raw[15] >> 2) & 0x3FFFF;
      parsed[7] = (raw[15] << 16) + (raw[16] << 8) + (raw[17] >> 0) & 0x3FFFF;
#endif

  for(i=0; i<8; i++) {
    parsed[i] = fixSignBit(parsed[i]);
  }
}

long fixSignBit(long reading) {

#if RESOLUTION == 14
    if(reading & 0x2000) { // if reading is < 0 (stored as two's complement)
        return reading | 0xFFFFC000; // set bits 31-14
      } else {
        return reading;
      }
#endif

#if RESOLUTION == 16 // 16-bit code not tested yet
      if(reading & 0x8000) { // if reading is < 0 (stored as two's complement)
        return reading | 0xFFFF0000; // set bits 31-16
      } else {
        return reading;
      }
#endif

#if RESOLUTION == 18
      if(reading & 0x20000) { // if reading is < 0 (stored as two's complement)
        return reading | 0xFFFC0000; // set bits 31-18
      } else {
        return reading;
      }
#endif
}

can you share me the shematic and pcb board for precision voltage shield ?