ADS7851: ADS7851

Hello Daniel,

In order to properly communicate with the ADS7851 via SPI, I suggest looking closely at Figure 1 from the ADS7851 datasheet. This is the timing diagram and if you follow the timing requirements for CS, SCLK, and reading from SDO-A and SDO-B, then you should be able to properly collect sampled data from the SAR ADC.

Also note that depending on the sampling rate you would like to run at, your SCLK can be adjusted. For example, at 2000kSPS you will need you SCLK to be at 32MHz to ensure you can capture data quickly enough per cycle. At slower sampling rate, the SCLK can scale to a slower frequency.

Please let me know if you have any follow-up questions. If so, please also inform me about the sampling rate you want.

Thank you,
Reed Kaczmarek

Hello Kaczmarek,

I'm expecting 1.5MSPS, which then needs clock speed of 24MHz(I believe it's correct, since one data needs 16 bits(2 dummy+14 resolution bits)

and here is the code I've used in Raspberry Pi

#!/usr/bin/python
import time
import sys
import spidev
import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
spi=spidev.SpiDev()
spi.max_speed_hz=24000000

SCLK=23
CS=24
MISO=21

GPIO.setup(SCLK,GPIO.OUT)
GPIO.setup(CS,GPIO.OUT)
GPIO.setup(MISO,GPIO.IN)

def readADC(clkPin, csPin, misoPin):
GPIO.output(csPin,GPIO.LOW)
buff=[]
for i in range(16):
buff.append(GPIO.input(misoPin))
bData=buff[2:16]
bData.reverse()
Data=0
for bit in bData:
Data=(Data<<1)|bit
return Data

if __name__=='__main__':
try:
while True:
val=readADC(SCLK,CS,MISO)
volt=val*3.3/(2**14)
print 'ADC value', str(volt)
time.sleep(0.1)
except KeyboardInterrupt:
GPIO.cleanup()
spi.close()
sys.exit(0)



Here, I've used time.sleep(0.1) , just to check the real-time voltage signal (Later I'll get rid of it to get full sampling rate)

Is this the proper approach of SPI Communication?

Thanks,
Daniel