0
$\begingroup$

I am learning about ADC MCP3422 Chip, because I want to use ADC Pi chip with my Raspberry Pi which is using MCP3424 ADCs, but I am having trouble understanding how the ADC calculation is done in the datasheet.

(Data sheet links are available in the end)

From what I know of ADCs,

An ADC has a bit size. and ADC tends to generate the total number of level based on the Reference Voltage.

for example,

If there is a 3-bit ADC then and the Reference Voltage is 2volts, then,

2^3 = 8-bits

Now 2volts will be divided in 8 levels.

8/2 = 0.25volts (each level will represent 0.25 volts

and,

000 = 0 -> 0.25 volts
001 = 0.25 -> 0.50
010 = 0.50 -> 0.75
011 = 0.75 -> 1.00
100 = 1.00 -> 1.25
101 = 1.25 -> 1.50
110 = 1.50 -> 1.75
111 = 1.75 -> 2.00

Now, in case of MCP3424, I have so much confusions

The bit size is 18. Reference voltage is 2.048volts

2^18 = 262144

Does that mean there will be 262144 levels ?

So, 2.048/262144 = 0.000007812volts Does that mean each level will represent, 0.000007812 volts ?

Also, the data sheet says,

The data rate for analogue to digital conversions is 3.75 (17 bit), 15 (15
bit), 60 (13 bit) or 240 (11 bit) samples per second. Data rate and
resolution can be configured within software using the I2C interface.

How the data rate is calculated here, and how come reducing the bit is increasing the Data Rate ?

MCP3424 Datasheet: www.pridopia.co.uk/pi-doc/mcp3424.pdf‎

ADC Pi Datasheet: http://www.abelectronics.co.uk/docs/stock/raspberrypi/adcpi2/Datasheet-ADCPiV2.pdf

$\endgroup$
1
$\begingroup$

If the conversion is 18-bit, there will be 262144 levels.

However, it seems to me that on the ADC Pi board, one of the differential inputs of each MCP3424 converter channel is grounded ; so this actually halve the range, and the first bit of a word read from the ADC will always be zero. That's why the ADC Pi datasheet reports half the resolutions compared to those published on the MCP3424 datasheet (17-bit, 15-bit, 13-bit, 11-bit).

There is a relationship between data rate and bit rate because the converter has a delta-sigma architecture. There is an internal conversion/integration clocked at a fixed 983040 Hz, and this conversion "budget" can be used at your whim to achieve different data rate / bit-rate ratios with:

$$s_r = \frac{983040}{2^{b_r}}$$

Intuitively (and this is a gross mis-simplification), reducing the sample rate gives the ADC more time to extract the finer details of each sample.

$\endgroup$
  • $\begingroup$ thank you , it cleared a lot, also, the data rate for 17 bit is 3.75, but 983040/2^17 gives 7.5. which is twice of 3.75. $\endgroup$ – Sufiyan Ghori Jan 12 '14 at 18:54
  • 1
    $\begingroup$ There's no 17-bit mode on the MCP3424. There's a 18-bit mode with a rate of 983040/2^18 = 3.75. But since the ADC Pi board is designed in such a way that it discards the first bit, you only get 17 bits out of these 18 bits. $\endgroup$ – pichenettes Jan 13 '14 at 2:36
1
$\begingroup$

So, 2.048/262144 = 0.000007812volts Does that mean each level will represent, 0.000007812 volts ?

MCP3424 has differential input, it can take an input of -2.048V to +2.048V.

Therefor volt per level = (4.096/2^18) = 0.0000625 => 62.5uV per level

Also, bit 18 is the sign bit.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.