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Tool/software: Code Composer Studio
On my custom board I use SD24 to acquire a differential signal on A0.0+ and A0.0- inputs. The A1.0+ and A1.0- inputs are both tied to GND. VCC = 3.3V.
I use a nominal 1.25V external reference voltage to feed the input signal and the VREF input of SD24 (ratiometric acquisition). The measured reference voltage is 1.266V.
SD24 is initialized as follows:
SD24CTL = 0; // External ref SD24CCTL0 = SD24DF | SD24IE; // 2-compl. data format, continuous conv, enable interrupt SD24INCTL0 = SD24GAIN_8; // input A0, Gain=8, Interrupt on 4th sample
When I start conversions with
SD24CCTL0 |= SD24SC; // Set bit to start conversion
I count 8 conversion interrupt; in each conversion interrupt I accumulate the (short int)SD24MEM0 value to make an average conversion value at the end of the 8th interrupt, when I stop conversions with
SD24CCTL0 &= ~SD24SC; // clear bit to stop conversion
I know from the MSP430I2021 datasheet the SD24 gain tolerance (±1%) and the SD24 max offset error (2÷4 mV) in the external reference case with SD24OSRx=256. I suppose a 2.5mV offset error with gain 8. The 2.5mV offset error would give (2.5mV * 8 * 32767)/1.266V = 534 ADC offset value added to every (short int)SD24MEM0 conversion result. Indeed, with a given input signal on A0 input I see conversion values differ from the expected ones by 500-650 ADC counts.
Now, in order to compensate that SD24 offset error I tried to acquire the GND signal tied to A1 input channel with the same A0 input channel configuration. The (short int)SD24MEM1 conversion result should be (more or less) the SD24 offset error to be subtracted from the (short int)SD24MEM0 conversion results of the A0 input signal.
But in this way I get 39 as measured SD24 offset error. Also, I get 82 as conversion result with A0 signal at 0V (without compensation).
Could you please explain the reason why? Where am I wrong?
Hi Lixin,
I don't see the problem for the software configuration. You discard first 13 conversion data and make average for 8 conversion results.
OK, but on the MSP430I20xx Family User's Guide I see that just 4 conversions (digital filter settle time) are needed to have an ADC result stable in any case. Maybe, this is true only for GAIN = 1 ?
1. Could you help to check if the the external Voltage reference voltage is stable or not? Is there any de-coupling capacitor on VREF pin?
2. Do you have tried internal voltage reference and see difference? Datasheet recommend capacitor on VREF pin is 100nF when using internal voltage reference. Please make sure the board has the capacitor.
Yes, the external voltage reference is 1.266V stable. I measure the same voltage on the VREF pin, which is de-coupled with a 100nF capacitor.
I cannot try internal voltage reference: I should trim the net on the VREF pin on my board.
3. Another thing needs to be checked: datasheet recommend to connect unused pin to AVSS. Could you help to check it?
I use only A0 input in differential mode with the following circuit:
A1, A2 and A3 inputs are all tied to AVSS.
4. When ADC conversion, it is better no GPIO toggling and no high frequency clock toggling because this will cause noise on ground. Could you help to check this?
On my board I blink a led with Ton=Toff=1s. I get the same results with no led blinking.
I use MCLK = DCO/2 = 8.192MHz, SMCLK = DCO/4 = 4.096MHz, External Resistor as clocks configuration.
I get the ADC conversion results through my debugger, stopping the firmware with a breakpoint at the end of each average calculation. I use Spy-by-Wire (2-wire JTAG) program/debug interface with my MSP-FET430UIF v1.4a. Could the debugger presence affect the ADC conversion results?
Best Regards
Alessandro
Hi Lixin,
I try to attach my complete analog input circuit.
So, you can see A0 is the used differential input, A1 input is tied to AVSS.
SD24 offset error on A1 was achieved converting A1 input with the same gain and average procedure used on A0 input. Because A1 is tied to AVSS, I suppose (am I right?) A1 conversion value could be the compensation offset value to be subtracted from every A0 conversion value. But when I set Vin = 0V (i.e. RTD = 0 Ohm) on A0 input, I read a much greater value then the compensation offeset value.
Best Regards,
Alessandro
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