Problems with multiply - AD1940 on SigmaStudio

Question

I'm using two sources, one DC with 28.0 and a sine tone, then multiplying it and using the real time display (in 28.0 format) to see the result. The negative numbers are represented as the maximum positive value. 
 What am I doing wrong? 
 Also, when adding 1 to the sine, and multiplying this signal by a small integer number (20-100), the result doesn't seem like a sine at all. I'm trying to modelate a delay for effects like Flanger, since the Controlled Delay takes only integer delays, I'd like to know how I can do it? 
 I'm starting with the SigmaDSP and I'm a little (very, actually) confused since I'm used to "usual" DSPs and programming with lines... 
 Sorry for bad english, not a default speaker.

KJBob · Accepted Answer

Hello GustavoKF, 
 Sorry I didn't understand your application -- I thought you were looking at audio tones, not the low-frequency modulation of your flanger. With the latter, there's no trouble with the frequency, but as Brett noted, the -1940 truncates the lowest 4 bits of the readback -- thus a resolution problem. 
 With hardware handy I tried the multiplication and initially got the same strange result you described. After some experimenting, I concluded that the Real-Time Display doesn't work with settings of 24.4 or smaller -- likely because the -1940 doesn't provide the full number of bits down there. Apparently the sign bit is also lost, causing the polarity flip we both see. To work around this I multiplied the display's input by 32, effectively shifting 5 bits to the left -- thus converting 28.0 to 23.5 format, which the -1940 can faithfully read back: 
 
 The Real-Time Display works fine at the 23.5 setting, as shown below: 
 . 
 Hope that helps, 
 Bob

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Problems with multiply - AD1940 on SigmaStudio