ADS8558 Internal Reference

Yes i am using in HW mode and i have grounded it properly. I have tied AGND to BGND as in i am having common ground. I  did check it on a oscilloscope and i am getting a sine wave having pk-pk voltage 2V. Is this output modulated one as in if i put a capacitor of some another value will it affect the output on REFIO pin??? 

My second doubt is that i want to use the ADC for range of 0 to 5 volts. So it should be better to give 5 Volts External to the REFIO pin, Am i right over here??? And yes i am using expansion board which i specifically made for mounting the LQFP package which is sort of similar to DIP package as shown in Proto-type site.

Hi Vishal,

You need to add a cap for the reference to be stable.  The ADS8558 cannot accept a 5V reference - the input to REFin would need to be under the 3.025V level.  The analog input of this device is bi-polar, so you would only be using half the available input range running with 0-5V on the input.  Please see the Analog Inputs section of the data sheet on page 23.

Yes i have added a CAP there but not of the value specified in datasheet i will add that by tomorrow and yes i got the point....thank you :)

Hi Tom,

I did add the CAP and i have kept RERen pin as high which means that am using internal 2.5 volts. My input range is 0-5volts,as you said its half of the total range. Like in many ADC ref voltages sets the range as in the output range for example if i give ref as 2.5 volts than step size would be 2.5/2^12 or if i give 5 volts it will be 5/2^12 and max input would be 2.5 or 5 for the above mentioned cases, i hope i am right here, In this case i am having 2.5volts as ref and my input is from 0-5volts my only doubt is that  how will the output react as in i am trying to find a relation between ref voltage, my input range and the output?? 

Thanks a tonne got it.....

Hi Tom,

I have connected Refen pin to ground and i am giving 2.5 volts external to REFIO pin. Initially i used to use internal 2.5 volts reference. For the time being i am using A0 channel only, and am giving pulse to Rd and Conv_a pins using switches (CS pin is always grounded). I used to get the output initially but since i have connected REFIO pin external 2.5 volts i am not getting the output. I have tied both the grounds. I have grounded har/sw pin also the par/ser pin. Can you think of anything which i am missing over here???. I have also connected STBY pin to BVDD. 

Range = BVDD

Word = BGND

HVSS = -5Volts

HVDD = 5Volts.

The capacitors are connected as per the datasheet.

I am giving high pulse to COnv_a first and then lowering the RD pin to get the output..

Hi Vishal,

My appologies for the delay in getting back to you - please try and monitor your CONVST impulse with a scope.  You may be putting the ADC channel in partial power down mode which could happen if your switch is giving any 'bounce' to the input signal.  Verify the CONVST against the BUSY signal and see if there are any transitions that the ADS8558 might interpret as a secondary CONVST input while BUSY is high.  If this is the case, you will need to toggle the RESET pin or cycle power to the ADS8558.

Hi Tom,

I have got the parallel output of all the six channels and my range is from -2Vref to +2Vref where Vref is internally supplied. Now the next task is to interface ADC with Virtex2pro FPGA. I am thinking of using a DCM and lowering clock so that i can give pulse to Conv_x pin and taking the parallel output on FPGA I/O pins.Will this strategy work out? If not than can you suggest me some material or files from where i can get information regarding this.

Thanks

Hi Vishal,

It should work, I can't think of anything off the top of my head that might pose a problem.  We don't have anything readily available for the Virtex2Pro, the EVM we have for the ADS8568 uses the Spartan family (we use the XEM3010 from OpalKelly).  We can provide you our source VHDL if you think that might help.

Thanks, I will try that soon. Yes it would be better if you could provide me with source code. That might help me get started on this. My email address is mehta.vishal.360@gmail.com

Thanks a lot

Hi Greg

Thanks for the code...I have made one using VHDL and am seeing the output on a MSO. Can i post/mail  the code and the output and can you verify it if it is possible?.

Hi tom,

Sorry for troubling you again but i am planning to write device driver  in  linux for ADS8558. Can you please send me the addresses of the registers/hardware module?. And on the same note i am also trying to write device driver for DAC7724 can you please send me the addresses of the registers of the DAC too?

Vishal -

For the ADS8558, please see the datasheet information on reading and writing to the configuration register (located at http://www.ti.com/lit/ds/symlink/ads8556.pdf ).

For the DAC7724, you can view the datasheet at http://www.ti.com/lit/ds/symlink/dac7724.pdf.  This device does not have control registers, so the part is controlled using external pins and the parallel data bus.

Hello again,

 I have been using the ADS8558 since a long time now...It was working very well but since last two days it is giving me constant output even though the analog input is changed.I saw if there was any short between the outputs but there is not and rest of the circuit is same.

I am using it in Hw mode with parallel output. I have kept CS as ground and am giving convert high  while read/write as low . I checked the busy signal output on MSO i am getting the output also but the parallel output remains constant even thought the input changes...

can you suggest what may be wrong???

Hi Vishal,

What is the state of your RESET pin?  if you toggle it high/low, does the ADS8558 provide the expected output data?

Initially when i was using i never gave anything to reset than also it used to give expected output. I made a new circuit yest and tested it and have included reset high/low also again the same result output remains same...and i guess its not the output of the analog input it is some random output which it gives. The thing is that i have a demo shortly and i thought everything is working fine till it stopped working so i am in a mess right can you help me in any manner?

Thn i thought may be this chip might be behaving in this manner so i changed the chip also but again the same output remains constant only....

Vishal -

Are you keeping to the timing with respect to Figure 3 and 4 in the datasheet?  You made the comment about setting CONVST while RD/WR are low...both of those signals are active low so you would want them to be high when issuing the CONVST.  You might want to verify this on your timing to make sure you these pins are set according to the previously mentioned datasheet figures.

I think so i am using it correctly since its the same code which made it work few days back.

my code is :(pins 0 and 2 are used for read and conv respectively)

 PORT = 0x05;

delay of 100 ms

PORT = 0x04;

delay of 100 ms

PORT = 0x05

delay of 100 ms

PORT = 0x01;

delay of 100 ms

Should i try Software mode????

Hello,

Finally i got the output now and the IC is working fine ...thanks a lot

Hi, 

May i ask you to have a copy of the VHDL code you use to drive the ADS855x in order to compare it with mine? Have you tried to drive the ADS855x at their maximum sampling frequency?

Thanks and best regards,

Goodmorning,

I'm just about to start a new data acquisition project in which i will use the ADS8568 driven by an FPGA ( i still haven't chosen which one to use ), is it possible to have

a copy of your VHDL code for this converter  in orfer to have s tarting point for my project?

Thanks in advance for your attention.

Regards.

Daniel.

Daniel -

Can you please start a conversation with me in which you include your email address? (This will prevent your email disclosure on the forum).

Once we have this information, we will be happy to provide you with the requested reference code.

I can give you the code but its a very rudimentary one if its fine with you...else yes code given by Greg is way better than mine...

Thank you very much Greg. I'll start it immediately.

Regards,

Daniel

Thanks for your concern Vishal.

Your code will be hepful too if you want to share it.

Regards,

Daniel.

HI Daniel,

  I am also using this ADS8568 ADC with spartan 3 FPGA. I have done Schematics And now i want to write verilog code  for the reading data.

 Could you please help me by providing the vhdl code to me if possible.

Thanks and regards

Sumit Sharma