driving ADS1255 whit pic18f4550 across SPI comunication

Hi Jose,

Do you have a specific question about reading and writing to the control registers in the ADS1255? Pages 34-37 in the datasheet explain the SPI commands.

If you want to use a different part without registers, look at the ADS1251, ADS1252, ADS1253, or ADS1254.

Regards,
Chris  

Hi Jose,

The control registers on the ADS1255 give you many options and functions built-in to the ADS1255. 

For example, the control registers can change the data rate, the PGA gain, select the multiplexer inputs to the ADC, modify the GPIO pin behaviors, and offset and gain calibration settings. These may or may not be beneficial to your specific application. Suppose you have multiple input sensors with different signal levels and data rates required, all of these controls would be very useful.

Another nice integrated function of the ADS1255 is the internal programmable gain amplifier (PGA). Without the PGA, the ADC input impedance is much lower. Therefore, if your input signal is from a sensor with high output impedance (i.e. it cannot drive a lot of output current) then you will see measurement errors resulting from an effective resistor divider between the source's output impedance and the ADC's input impedance. The ADS1251-ADS1254 may require you to buffer your signal before connecting it to the ADC. The ADS1250 is another simple alternative device that does include the internal PGA, but it's gain can be controlled externally with pull-up or pull-down connections.

So a lot depends on your application... What are you trying to measure and at what data rate/gain do you need?

Regards,
Chris

hi Christopher !!!!

Ok! I already see the advantage of the control registers. I am going to tell you what I'm trying to do.

I will measure deformation from the load cell whit the ADS1255. the porpouse is make a test to count the fatigue cycles that iron sample (or another material) can bear.

the most insteresting is make a sweeping of  cycle frequency.

the cycles are of   stress (<------->) and  compression (>----------<).

the maximum frecuency is 100 Hz.

the ADS1255 can help me whit high resolution and filtering noise.

what do you think? I'll need this device or other most simple.

Hi Christopher !!!! tanks so much for your referred devices earlier .

I've already them and They looks afther SO WONDERFUL!!! and  so all i have to do is set them with external control signals

so I guess the best is the DAS1254 What do you think?

tanks so much for your time and you reply!!!!

best regards!!!!!!

Hi Jose,

The ADS1254 sounds like it will be the easiest for you to work with.

The ADS1254 is not single-cycle settling like the ADS1255. This is not a problem though, it just means that you need to be aware of the internal digital filter behavior if you have a step change on the input signal, such as when you switch to a different MUX input. Read the "Digital Filter" description on pages 8 and 10 of the data sheet for a detailed description. However, in summary...

...the data converter is oversampling and averaging previous conversion results to help achieve low noise results. If then your input changes quickly, the data converter will still be averaging in the old data prior to the input change. To make sure that your data is up-to-date, ignore the first 5 conversion results after a step input change (or channel change) to make sure that the data is "settled". Single-cycle settling devices always take a new 5 (or X) number of samples (hiding the intermediate results) to provide (what is hopefully) settled data. I say "hopefully", because the input signal can still change during those samples and the single-cycle settling devices will then have the same problem.

I hope I explained that clearly!

Regards,
Chris

HI!!! Christopher tanks so much!!!! :)

Ok If the PIC makes a change of channel I most ignore first 5 results, because the filtering/desimation module still working with old data. 

but if the frecuency signal change quickly  i have to make the same... hide the first 5 results...... or unless the frecuency is under of  20 KSPS in case of ADS1254 that isn't problem. is this right?.

and the data sheet makes mention about the diferential input signal, It means that the range signal is for example from 2 V to gorund,I can connect 2V to (+ch1) and ground to  (V-ch1) .

Ihope that you have a good day!!!!! Christopher

Hi Jose,

Sorry for the delay. The input signal in that case would be attenuated by the digital sinc filter. Since the first notch in the digital filter response would be 20 KSPS, frequencies near 20 KSPS would be highly attenuated while lower frequencies would appear more like a flat passband. See pages 8-10 in the ADS1254 datasheet for more information on the digital filter and an equation with it's transfer function.

Keep in mind the settling time of any signal conditioning in your circuit. If you use large RC filters between the sensor and ADC those may not settle quickly. For best accuracy, make sure the capacitor charges to within 1/2 LSB accuracy of it's final voltage and then wait 5 conversion cycles before accepting the ADC measurement.

You can also monitor the output code of the ADC and see if gradually changes from reading-to-reading to know if your data is unsettled. Over time the output code should settle to a gaussian distribution around your input voltage, with the standard deviation resulting from the amount of noise in your circuitry.

Regards,
Chris

Hi!!!! Christopher!!!!! don't worry!!!! :)

tanks so much for you time and your reply!!!!

I've read the data sheet and I got why set the output data rate to attenuate noise above the interest bandwidth ( from .2035output-Data rate ---- to ---- 20.8/2 KHz if the system clock is 8MHz ) , the digital filter response shows how it attenuate the noise gain.

The DVdd is suplied by USB that  means from 5 V to D ground ..... but trouble is the ADS1254 accepts from 1.8 V to 3.6 V ..... :S  do you know how to get 3 volts for ADS1254 DVdd?

tanks so much for your reply

best regards!!!!!

have a nice day Christopher......

Hi Jose,

You can consider using a resistor voltage divider or LDO to supply DVdd.

Regards,
Chris