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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 29th May 2016, 05:11 PM   #1
Frex is online now Frex  France
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Join Date: Nov 2009
Arrow SAR ADC for high performance audio ADC project [LTC2380-24]

Hello all,

As many DIYers here and from my own experience in building ADC's,
we can seen that no obvious improvement was done from much years in ADC IC available.
The AK5394A designed 12 years ago still the better audio ADC available on the market.
I really think that the main reason come from the lack of market for a really better performance.
In all my investigations, i seen nowadays that SAR ADC market mainly used for instrumentation
have reached a performance level that become comparable to the better sigma/delta audio ADC.

LT has released some months ago a new SAR ADC, the LTC2380-24.
It is an high resolution 24bits 2Msps ADC that integrate a digital averaging filter to improve SNR.

After many hesitation, i had decided to purchase the evaluation board of this IC.
The eval board is intended to be connected to a DSP board (from LT also) for data collecting
to a PC and analysis with the LT software ( PScope).
I don't want to buy the DSP board because it's pretty expensive(300€) and my target
is to use the EVM with all my favourite audio software for sound-card.

The EVM board include an FPGA (EPM570) that is used to sent data to DSP board.
The ADC use a serial link (as SPI) to enable conversion and read serial data.

So, i wrote a complete new software in the FPGA to read the ADC at 1.536 MSPS,
and average 4,8,16 or 32 samples to give an output at standard audio rate of 384kHz, 192kHz, 96kHz and 48kHz.
Then, the data rate is transformed to fit in directly in SPDIF format,
so CPLD outputs drive a pulse transformer to get a coax SPDIF out.
At rear, 3 positions toggle switch allow to change sampling rate or perform a DC calibration
(the ADC do not have HPF has many audio ADC have).

The EVM need also an external clock, that must be low noise and high frequency
to get maximum sampling rate and minimum jitter.
So,i add a 98.304MHz ultra low phase noise oscillator(Abracon ABLNO)
powered with low noise 3v3 regulator.

The original input front-end of the EVM is very simple, with only followers.
The ADC have differential inputs with Vref/2 offset, so the inputs must have this offset (not friendly).

Because i want to use inputs in single-ended or differential mode, i designed another
front-end including a little low-pass filter to limit aliasing and out of band noise.

After some investigations and tests, I designed a front-end using the MAX44206
fully differential amplifier. The buffer has unity gain and allow 10Vpp full scale input (3.5Vrms).

After all of this done, i was very curious to know what we can really expect from this type of ADC...

First , you will find below some pictures of the setups with the EVM becomed
an audio ADC.
Then, some measurements done today with it.

Front view with differential input and on/off switch
Click the image to open in full size.

-1dB 10kHz THD test with EOSC10KV3 oscillator
Click the image to open in full size.

Top view, EVM PCB with oscillator and front-end buffer
Click the image to open in full size.


Now, some measurements results.

1/ 192kHz mode noise floor, 50 Ohms on each input (orange trace).
Green trace is the noise floor of AK5394A ADC to compare them.
Click the image to open in full size.

We can see here that despite the lack of noise shaping and only
low oversampling ratio of LTC2380-24, the noise floor level is very near
what we get with AK5394A (-111dB).
The floor of LTC2380-24 is also extremely flat over bandwidth.

2/ 1kHz THD at -6dBFS 192kHz, single-ended.
Generator is EOSC10KV3 1kHz version, output level set to 3.15Vrms (8.9Vpp)
Click the image to open in full size.

3/ 10kHz THD at -1dBFS 192kHZ, single-ended.
Generator is EOSC10KV3 10kHz version, output level set to 3.15Vrms (8.9Vpp)
Click the image to open in full size.

4/ 10kHz THD at -21dBFS 192kHZ, single-ended.
Same measurements as previously, but -20dB passive attenuator added
between generator and ADC input.
Click the image to open in full size.

5/ 10kHz THD at -41dBFS 192kHZ, single-ended.
Same measurements as previously, but -40dB passive attenuator added
between generator and ADC input.
Click the image to open in full size.


As we can show, measurement results are very far to be ridiculous !
When can see exceptionally clean spectrum with THD level excellent at any levels,
even near to full scale. So, these results confirm me that modern SAR ADC IC
can be a very good choice to build a high performance audio ADC.
There is also many others advantages that is :
High DC accuracy (could be usable as high resolution voltmeter).
Easy to get 384,768 and event 1536 kHz sampling rate ! (if supported by sound-card).
DNR can reach 145dB using sample averaging (low rate).

I will continue to investigate in this direction, and i will also probably try the EVM
with the LTC2378-20 (pin compatible) that claim a better THD figure than the LTC2380-24...



Frex
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Old 29th May 2016, 09:01 PM   #2
JensH is online now JensH  Denmark
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SAR ADC for high performance audio ADC project [LTC2380-24]
Hi Frex,

A very interesting project. And the performance seems to approach that of the AK5394A.
These new converters are definitely on my radar when looking for solutions capable of providing a good performance and sample rates above 192kHz. At the moment I am quite busy trying to finish my AK5394A based Analyzer, but after that I hope to get a chance of looking at the higher sample rates.

One of the challenges with the SAR converters is that they require a good external low pass filter.
How much filtering did you use?
Did you bypass the ADC driver on the board or did you build an amplifier in front of it?
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Old 29th May 2016, 09:56 PM   #3
ruairi is offline ruairi
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Very interesting Frex! Do you have the ability to do IMD measurements? 19 & 20Khz for example?
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Old 29th May 2016, 10:21 PM   #4
CharlieLaub is online now CharlieLaub  United States
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SAR ADC for high performance audio ADC project [LTC2380-24]
Hi Frex,

Wow, this is a very interesting project. I am doing loudspeaker crossovers under Linux and have been looking for a 96kHZ or higher sample rate ACD to get the signal into the hardware so that I can process it. I can already get spdif into the computer, and this fills the missing link of analog --> SPDIF. The performance is certainly more than adequate!

Please keep working on this and post your progress!
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Old 30th May 2016, 12:36 AM   #5
dontbugme is offline dontbugme  Australia
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This is hugely significant. Very nice work.
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Old 30th May 2016, 12:57 AM   #6
soekris is offline soekris  Denmark
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SAR ADC for high performance audio ADC project [LTC2380-24]
Great work, the LTC2380-24 is even better than I expected it to be....
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Old 30th May 2016, 02:13 AM   #7
ruairi is offline ruairi
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A question if I may? Does averaging 10 times on the FFT not render the noise measurements incorrect?
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Old 30th May 2016, 02:55 PM   #8
HpW is offline HpW
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Bingo,

may you consider to build a ADC module for Jens upcoming Analyzer, while his Analyzer is on a modular concept and may supporting higher sample rates

Hp
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Old 31st May 2016, 06:59 PM   #9
Frex is online now Frex  France
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Join Date: Nov 2009
Quote:
Originally Posted by JensH View Post
Hi Frex,

A very interesting project. And the performance seems to approach that of the AK5394A.
These new converters are definitely on my radar when looking for solutions capable of providing a good performance and sample rates above 192kHz. At the moment I am quite busy trying to finish my AK5394A based Analyzer, but after that I hope to get a chance of looking at the higher sample rates.

One of the challenges with the SAR converters is that they require a good external low pass filter.
How much filtering did you use?
Did you bypass the ADC driver on the board or did you build an amplifier in front of it?
Hello JensH,

You are right, to avoid aliasing you need a good low pass filter.
Actually i use only 2nd order filter built with the fully differential input buffer (Fc=90kHz).

I work to improve that situation using high performance FIR filter using interpolation and decimation.
That allow to reduce drastically the input filter because of 8x oversampling ratio (1536kHz /192kHz).
This can be done in different ways, using the DIF1706 that make all the job but seem to be obsolete.
Another approach is to make on in CPLD. I work in these two ways.


ruairi,

I don't have already done IMD tests, but i will do that soon.
So, i have only 10kHz and 1kHz ultra-pure oscillators, so i will try with this mix.
And no, the FFT averaging doesn't change the effective physical noise floor.
Averaging allow to reduce random noise for harmonics and spurious extraction.
(SNR still unchanged).

With the ADC I also made new THD measurements of my EOSC10KV3 oscillators (from LT AN67) and my notch filter gain box set-up.
Results confirm well that all harmonics of the oscillator are below -140dBc (0.1 pmm).

Frex

Last edited by Frex; 31st May 2016 at 07:12 PM.
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Old 1st June 2016, 09:06 PM   #10
chris719 is offline chris719  United States
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SAR ADC for high performance audio ADC project [LTC2380-24]
I'm a little confused as to how the DF1706 would help? You need to decimate not interpolate since you are already oversampling, no?

As you mentioned, a filter in an FPGA or DSP would do the trick.
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