<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><div class=""><div class="">The circuit is straight out of Microchip’s datasheet and is what they recommend. But that’s not to say that it can’t be improved!</div><div class=""><br class=""></div><div class="">Which Douglas Self book did you mean? He’s written a few.</div><div class=""><br class=""></div>
<div><br class=""><blockquote type="cite" class=""><div class="">On 5 May 2021, at 11:11, Mike Bryant <<a href="mailto:mbryant@futurehorizons.com" class="">mbryant@futurehorizons.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="WordSection1" style="page: WordSection1; caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class=""><span style="font-size: 11pt; font-family: Calibri, sans-serif; color: rgb(31, 73, 125);" class="">You probably want to use something a little more complicated for the best performance. Douglas Self covers this well in Chapter 26 of his book.<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class=""><span style="font-size: 11pt; font-family: Calibri, sans-serif; color: rgb(31, 73, 125);" class=""><o:p class=""> </o:p></span></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class=""><span style="font-size: 11pt; font-family: Calibri, sans-serif; color: rgb(31, 73, 125);" class=""><o:p class=""> </o:p></span></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class=""><span style="font-size: 11pt; font-family: Calibri, sans-serif; color: rgb(31, 73, 125);" class=""><o:p class=""> </o:p></span></div><div class=""><div style="border-style: solid none none; border-top-width: 1pt; border-top-color: rgb(181, 196, 223); padding: 3pt 0cm 0cm;" class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class=""><b class=""><span lang="EN-US" style="font-size: 10pt; font-family: Tahoma, sans-serif;" class="">From:</span></b><span lang="EN-US" style="font-size: 10pt; font-family: Tahoma, sans-serif;" class=""><span class="Apple-converted-space"> </span>Synth-diy [<a href="mailto:synth-diy-bounces@synth-diy.org" class="">mailto:synth-diy-bounces@synth-diy.org</a>]<span class="Apple-converted-space"> </span><b class="">On Behalf Of<span class="Apple-converted-space"> </span></b>Tom Wiltshire<br class=""><b class="">Sent:</b><span class="Apple-converted-space"> </span>05 May 2021 10:17<br class=""><b class="">To:</b><span class="Apple-converted-space"> </span>synth-diy francesco mulassano via<br class=""><b class="">Subject:</b><span class="Apple-converted-space"> </span>[sdiy] Differential amps and noise cancellation in dsPIC 33FJ128GP802 uP's DAC<o:p class=""></o:p></span></div></div></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class=""><o:p class=""> </o:p></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class="">Hi all,<o:p class=""></o:p></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class=""><o:p class=""> </o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class="">The dsPIC 33FJ128GP802 uP (and others in the family) includes a 16-bit DAC on the chip. The outputs from the DAC are provided as a differential output to help cancel any noise generated by the uP internally. NOt a bad idea, seeing as that is probably inevitable. Despite this precaution, the DAC is reasonably noisy and not suitable for all applications.<o:p class=""></o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class=""><o:p class=""> </o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class="">The example circuit for combining the differential outputs is shown attached - a simple diff amp with 10K resistors everywhere.<o:p class=""></o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class=""><o:p class=""> </o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class="">Now, my question is this:<o:p class=""></o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class=""><o:p class=""> </o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class="">If I change the gain of the differential amp by increasing R6/R8 (to 47K, say) does this affect the noise cancellation properties of the diff amp?<o:p class=""></o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class=""><o:p class=""> </o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class="">It seems to me that it might, because the positive input now has a different proportion of signal going to it than the negative input.<o:p class=""></o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class=""><o:p class=""> </o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class="">Thanks,<o:p class=""></o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class="">Tom<o:p class=""></o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class=""><o:p class=""> </o:p></div></div><div class=""><div class=""><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class="">==================<br class=""> Electric Druid<br class="">Synth & Stompbox DIY<br class="">==================<o:p class=""></o:p></div></div><div class=""><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: "Times New Roman", serif;" class=""><span id="cid:image002.png@01D7419C.66312780"><image002.png></span></div></div></div></div></div></div></blockquote></div><br class=""></div></body></html>