ADS1256 Crystal oscilator start-up problems

Hello Tony,

Thanks for useful application note.

But I am still confused with yours application engineers choice of Cx1 and Cx2 capacitors in ADS1256 evaluation module.

They have used 18pF with crystal which has load capacitance CL=18pF. So, they have 9 pF from serial combination of Cx1 and Cx2, and 3~5pF form stray capacitance (According to equation 3 from attached application note).

In , best case they have total load capacitance of 14pF which is 4 pF less than crystal’s CL. Why the haven’t used 22pF instead of 18 pF or maybe 27 pF?  

Please, find bellow oscilograms of start up and steady state of crystal oscillator with different values of Cx capacitors. For all values of Cx, start up seems ok. I had 18pF by default when modules came from reflow soldering. 

Why they woudn't oscillate first time on power supply up when they came from reflow soldering, I don't know.  

Best regards,

Josko Marasovic

Hi Josko,

There are additional capacitance that you may not necessarily be accounting for in your design. You are going to have a couple of areas that stray capacitance can come from; a differential capacitance between the two XTAL pins, a capacitance from each XTAL pin to ground, and board capacitance coming from the layout and trace widths. This can easily be larger than the 3-5pF which you are stating accounts for the stray capacitance. For this reason, we recommend going with smaller capacitors, 10 - 15pF would work best. You mention that you have trouble with your crystal starting up the first time after the power supply comes up. After the first time, does it start up problem free? If so, your start up problem may be due to too large of capacitance, maybe the board needs to be cleaned as flux may be causing a problem across the crystal, maybe adding a little heat to the board (components heating) take the crystal a little time to start up. 

Regards,

Tony Calabria

Hi Tony,

About crystals starting up the first time after the power supply comes up, yes I had problem with crystal start up first time whith each module (I had 10 PCB protoypes).  After the first start up, problem had occured mybe one or two time and after that I tried to stuck crystal on start-up but I didn't  suceed. Now, each crystal on prototype module starts every time when power supply comes up. I have to admit that I have never had probelm with crystal oscillators as like as this.

There is on additional thing that I didn't mention. I have provided one additional parallel connection of clock source for ADS1256. It is 3.3V oscilator which is connected to the ADC's XTAL1 pin but it is not paleced of courese. Maybe these traces contribude to stray capactiance to being grather than 3-5 pF.

Please, find below TOP and BOT layout of my PCB. Parallel capacittors Cx1 andCx2 are below crystal pads. 

Btw, even it is not subject to the this topic, do you have any useful application note about op amps or In-amps input termination? I use INA326 amplifier in single supply 0-5V to measure RTD resistance in 4-w kelvin connection. I have RF filter on in-amp input but I haven't done any termination. Because of that, inputs of in-amp float when RTD isn't connected and ouptut of in-amp goes to VCC or GND.

 Best regards,

Josko

Hi Josko,

Thanks you for your feedback.

As for termination, I would recommend using a simple pull up or pull down resistor on the two RTD input lines (after the RTD but before the RC filter). If you would like to pull it to something other than VCC or GND, you can use a pull up resistor to a midpoint using a resistor divider. If you want to post a little snip of your schematic, I will take a look at it to let you know what I think.

Regards,

Tony

Hi Tony,

Your Welcome for feedback.

Please, find attached part of schematic for RTD input.

I am looking forward to your opinion.

Best regards,

Josko

 8357.Pt-100 interface.pdf

Hi Josko,

I attached an example of how we design in protection circuitry in a thermocouple applications. As you can see, in this example, we use pull up resistors to protect the analog input. In your application, you are using an excitation current source, so if you decide to use pullup resistors, you will create a current divider between the PT100 and the pullup resistor.

In your application, are you suggesting that you are going to disconnect the PT100 and the 4 connections to J2:A, J3:B, J3:C, J3:D? If so, then the INA326 is going to have floating inputs as you suggested and then looking at the worst case situation, the INA326 output will rail to either 5V or 0V (even if the gain is set to 36). Regardless of the additional ADC gain selected, the absolute maximum input spec is AVDD+0.3V so if the input is seeing 5V, sure the ADC will read back full scale, but you will not have to worry about damaging the ADC. One other thing to make sure is that the D17 diode you are using are not rated less than 1V. The reason being is that I believe your Iexct is 1mA and when it sets your bias across R43, you will have the V- node at 1V. You do not want this 1V to be fighting with the D17 diode turning on.

Otherwise, I do not believe that a pull up resistor is the right solution for you. If you are really concerned with the floating INA inputs, you may have to include a switch of some sort.

Regards,

Tony Calabria

Hi Tony,

Thanks for overview.

In my application, there is more than one Pt-100 input and not all of them will be used at the same time. There will be situations where we will use all Pt-100 inputs but it depends of application where we will use our DAQ card.

In case where only one Pt-100 input will be used,  all other Pt-100 inputs will be unused which means open (floating) inputs of INA326. This is what concern me and why I asked about input termination. I will consider your solution with switches but if there is no any reason to damage ADC input and INA326 by itself when the INA326 output goes to either 0V or 5V I will leave inputs to float.

I have made attention on stand-off voltage of protection diodes when I was choosing them. They have VRRM greater than 1V, but it seems that I forgot on reverse leakage current. All diodes have reverse leakage current about 2-4uA which probably contributes to error of current source.

Best regards,

Josko