Measurements and experiments carried out on a more spacious AK4490 test board
- Better understanding on improving AK4490 performance
- Encouraging results regarding Wee DAC performance goals
- Massive boost of 10-15 dB in THD+N at low frequencies
- THD+N at 1 kHz measured beyond -110 dB on test board
- Does not mean Wee DAC will achieve these though
- Modifications will increase BOM cost so need to consider the benefits and find a good compromise
Earlier measurements recap
Four weeks ago I wrote the first Wee DAC measurement update and here is a recap of it.
Initially I had issues because of powering the supplies directly from bench PSUs. This kind of indicated that something in the power stuff is quite sensitive here (at least indicates when thinking about it now afterwards..). By using W-PSU with on-board regulators, THD+N at 1 kHz went up to -99 dB. VREF references emerged to be the sensitive bits. When adding loads of capacitance on VREF lines, things improved significantly and the best THD+N measured was -108 dB. That is an excellent result; however, it was with two massive 2200 µF capacitors, see a photo of one below. Not very viable in this form factor, and not so elegant either.
And that figure was THD+N at 1 kHz. At lower frequencies things still were not great. I would assume THD+N vs. frequency graph to be nearly flat but it was far from that. More tests indicated that it was still an issue supplying VREFs.
Before moving forward, I also ordered a couple of different high quality polymer and tantalum capacitors to see if they would flatten the THD+N vs frequency graph, but they did not bring notable improvements over bulk capacitors.
AK4490 test board
Before re-spinning the W-DAC PCB, I wanted to make a more spacious test board that gives the opportunity to experiment a bit and see what is important around AK4490. The board has numerous supply options for VREFs, VDDs, and other supplies, using either LT3042 or ADP7118 regulator or opamp buffer for references.
Findings and performance improvements
I did quite a lot of tests; however, there are so many variables that not everything can be tried in reasonable amount of time. Here are some of the findings of these measurements.
These results focus on VREFs as these seem to be the main concern on AK4490. Quick tests showed that VDDL and VDDR are not that important although these should be of decent quality as well. AVDD does not really matter much and can be taken from DVDD supply.
Voltage regulators and buffers
First of all, Analog (ex Linear) LT3042 is a superb regulator and gives excellent results when used for supplying VREFs. Another benefit is that there is no need to use a massive electrolytic capacitor which is especially good on a small PCB. With LT3042 and solid layout THD+N at 1 kHz and near full-scale reached -111 dB. Moreover, THD+N vs frequency is almost flat now, giving huge improvement of 10-15 dB in lower frequencies compared to initial W-DAC results. This is what I hoped to see!
There is slight difference between left and right channels near full-scale level as right channel is wired from LT3042, while left channel has solid copper connections. These references are very sensitive and even a short and solid wire deteriorates the performance slightly, indicating layout may be critical as well.
I also tried ADP7118 as it is a lot cheaper than LT3042. However, the results were not that good although still better than the initial W-DAC results. I also tried LM4562 opamp buffer for references, as for example on AK4490 evaluation board an opamp-based buffer (with discrete components) has been used. Again, results were good but not as good as LT3042. I believe one can make an excellent opamp buffer with careful design using also discrete components. However, the overall cost would come close to LT3042 anyway and the solution would be a lot larger than the tiny LT3042.
Increasing reference voltage
While nominal voltage for analog supplies and references is 5 V, the maximum rating is 7.2 V. Therefore, you can find lots of discussions on forums people running these supplies with higher voltages and reporting better results. And it does bring some benefits but it is not that simple. I did my measurements with 6.65 V because it still allowed me to carefully use 6.3 V capacitors (and because using 100k and 200k resistors in parallel for setting LT3042 voltage just happened to give 6.65 V).
First of all, output level is directly proportional to VREFs. As noise level seems to remain constant, increasing signal level by increasing VREF boosts dynamic range. By moving from 5 V to 7 V one can expect around 3 dB boost in dynamic range – which is always beneficial.
THD+N is a bit tricker. Higher VREF re-shapes the curve quite a bit. This leads to lower distortion at lower signal level but higher distortion near full scale. For example, at -20 dBFS THD+N was 3 dB lower when using 6.65 V VREF. However, the increase near full scale is very steep, leading to 7 dB worse distortion at 0 dBFS. Compare the graph below to the one above. Which one to prefer is more like a design choice. I think I will keep the voltages at 5 V.
Towards new version of W-DAC board
These are only results of the test board. I will still carry out a couple of tests and then need to decide what changes do I implement on a new version of W-DAC board. It will still be a small board with restrictions in terms of real estate, other components, and also price.
I will use LT3042 as it is both very small, especially given it does not need huge electrolytics, and very good. The only downside is price – it costs more than the AK4490 DAC chip. Therefore, I do not want to use a separate IC for all four supplies as it would blow up BOM cost. A compromise is required here and I do not know yet whether I try to get along with only one regulator or should I use two. Anyway, there will be cost implications but with these performance improvements it is worth it.
References on AK4490 seem to be quite sensitive so the final W-DAC performance remains unknown until a new board is designed, built, and measured. I hope to get as close as possible to these test board results but there will likely be some deterioration, depending on how I will end up connecting the supplies, how many regulators to use, and layout of course. And always a pinch of black magic.