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SAR ADC for high performance audio ADC project [LTC2380-24]
SAR ADC for high performance audio ADC project [LTC2380-24]
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Old 14th October 2018, 04:07 PM   #201
descon is offline descon
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Join Date: Sep 2018
We have used the IL260 isolators for interfacing between the ADC and FPGA or Microcontroller and those work well. For slower speeds have used the HCPL2630 optoisolators. We monitor the current pulse into high power LEDs during testing to determine bonding to the substrate. We've used the INA103 (differential). Current speed (older design) is ~100ksps, but newer parts require from 250ksps to 1Msps. The LTC2500 is vastly configurable - averaging of 4 samples gives 250ksps and a range of 109db with 13uV of noise- if the front end circuitry is capable of equal performance.

The LTC2500 when sampling at 1Msps has an impedance of about 20K so buffer is still needed. Still trying to decide if this is best one to use.
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Old Yesterday, 04:33 PM   #202
Frex is offline Frex  France
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Join Date: Nov 2009
SAR ADC for high performance audio ADC project [LTC2380-24]
Hello,

I have received some transformers and i made some pulse response tests
using a DIY pulse generator with less than 100pS rise time and 20ns duration.

A picture of the pulse generator and the DUT test board.
Click the image to open in full size.


I tested three transformers (others coming but not yet received) ;
Scientific conversion SC947-02 (audio)
Wurth 749020100A (LAN)
Wurth 749010310 (LAN)

For all tests, signal level is set to about 500mVpp at transformer input
with impedance adapted to 75 Ohms on each side (I made this choice to be same
as SPDIF audio link, but that could be 100 or 50 Ohms).

One picture of the two wurth LAN transformer and the SC947-02 mounted
on DUT PCB.
Click the image to open in full size.

Click the image to open in full size.


Now the pulse test results (gray trace is generator alone as reference) :

SC947-02
Click the image to open in full size.

Wurth 749020100A
Click the image to open in full size.


Wurth 749010310
Click the image to open in full size.



These first results speak by themselves...
The last wurth LAN transformer work very well and output pulse rise time
look same as input and waveform is very clean.
We must note that this transformer is much cheaper than the SC947-02.

I will post soon others results, and of course i work a lot to complete
the AA2380 schematic in the next weeks.

To follow...

Frex
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Old Yesterday, 05:25 PM   #203
DPH is offline DPH  United States
diyAudio Member
 
Join Date: Dec 2008
Location: Portland, OR
Quote:
Originally Posted by descon View Post
The only method is by lowering the bandwidth to 5sps or less, and even then it's only 28 bits or so. We need as much resolution as possible for precision current monitoring at up to 1Msps, a bit different. Have used parallel amps to lower noise and moving window averaging, but need still better performance. Fortunately our source is low impedance - 5 ohms or less. The INA103 has excellent linearity but is pretty old now.
Search around for Gerhard Hoffmann's (username "gerhard") preamp designs for low noise PSU measurements. His stuff is probably the closest public domain (at least here, I haven't looked around the greater interwebs for heroic preamps) for getting a high ENOB-preamp at low source impedences.
__________________
Happy DIYing, Daniel
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Old Yesterday, 06:59 PM   #204
descon is offline descon
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Join Date: Sep 2018
Exceptional results, thank you. I have done high speed pulse generators (10ns) for laser diode work with somewhat different techniques, and have used terminated coax cable for very clean sub-nanosecond pulses by discharging the charged length of cable into the load.


Following earlier posts we are currently designing an interleaved system to achieve higher sample rates while minimizing noise. It does require an FPGA to move data to memory to achieve the highest rates.
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