[sdiy] Prophet 10 op-amp swaps?

rsdio at sounds.wa.com rsdio at sounds.wa.com
Wed Jul 31 22:30:03 CEST 2013

On Jul 31, 2013, at 12:47, cheater00 . wrote:
> Brian,
> On Wed, Jul 31, 2013 at 9:20 PM,  <rsdio at sounds.wa.com> wrote:
>> I recently designed a 32-channel DAC for ultrasonic frequencies
> really curious, 32-channel ultra-sonic DAC - are you allowed to tell
> what the application was? I imagine an array of transducers, possibly
> for measurement purposes, relating to mechanical modes of rigid
> bodies. Alternatively, a software radio but those normally just use a
> single output DAC for whatever transmitter you've got (usually just
> one!). I'm also reminded of a recent experiment I saw in ultrasonic
> levitation, the parts for that are less than $100, but that doesn't
> need a DAC, so this would have to be much more sophisticated...
> Cheers,
> Damian

The application is the Soundplane. Your hunch was reasonably correct:  
the Soundplane is a capacitive sensor that detects mechanical  
deflection of capacitive plates within a surface, in order to detect  
pressure and position. This sensor does not rely on human flesh for  
conductivity, and thus the surface can detect drum sticks or any  
material. In addition, the surface is flexible and can also bend. The  
ultrasonic signals really only go up to 60 kHz, with a sample rate of  
125 kHz.

To get back to the original topic, many op-amps have bandwidth in the  
range of 1 MHz or even 10 MHz, but that is for small signals. When  
dealing with 5 V power supplies and rail-to-rail op-amps, the full- 
scale bandwidth might be as low as 10 kHz! This is related to the  
slew rate of the op-amp, such that it can handle a small 1 MHz  
waveform, but cannot make the wide voltage swings of a 5 Vpp wave  
above 10 kHz. I found it rather difficult to get 60 kHz signals  
through an op-amp that was rated for 10 MHz, simply because the  
signals were amplified to a level that was too much for the op-amp.  
Price did not go up much, but I did have to search through quite a  
few op-amps, buying samples of each that met the specifications, and  
testing to see which ones really worked well in my exact circuit (a  
capacitive feedback amplifier with current input).

In Eric's case, the simplest solution might be to pick another op- 
amp. However, it might be very educational to examine the circuits  
schematics that are now oscillating, and then design compensation  
that would prevent oscillation. SPICE simulations could probably save  
a lot of trial and error, here.


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