Designing a high voltage MUX for low cost ultrasound applications - Page 1
Forum section: electronics / projects

author: helloThere
author_rank: Contributor
timestamp: July 06, 2025, 11:37:48 am
content: Hello all.You might've seen my previous post about designing a piezo driving/receiving circuit for my senior capstone. That worked relatively well!I had to do some rework on the previously posted design--we did full turnkey and that got it mostly working. Most of the issues stemmed from manufacturing.I was thinking about designing an ultra low cost ultrasound system. I wanted to get some input to see if my idea would work.My general idea is to take my driving/receiving circuit then put it through a high voltage MUX. So basically for each driver, it would be able to go to 8 PZT piezos.But I'm just uncertain abouthigh voltage MUXs. That's where the root of my questions stem from.I know there are specific MUX chips for piezos. I skimmed through the datasheets and I wasn't 100% on how they worked. They seemed relatively complex.MAX14866 looked like what I was looking for. But it's fairly high cost $48 for 16 channels.HV2801K6-G looks very similar but is 32 channels. Its price varies from like $24-$48. It looks like a series of analog switches rather than a MUX. You could just connect a voltage source to all the switches and then turn off switches to turn it into a MUX?Has anyone on here used similar chips? Just curious your POV? I know this is kinda adventurous and might be dumb, but I was thinking about designing a discrete solution.I think like a bidirectional SSR/two mosfets to allow for bidirectional current flow could be used. These components are extremely low cost, but of course I would need a lot of them so the cost might add up. I'm assuming these chips are doing something similar. Would this be a completely dumb solution?

author: David Hess
author_rank: Super Contributor
timestamp: July 06, 2025, 01:42:25 pm
content: What kind of voltage and current do you need?My first thought is to use old school diode multiplexing, which works on currents rather than voltages.Bidirectional MOSFET SSRs would work, but their high capacitance needs to be considered.Some integrated output circuits have a shutdown control which places the output into a high impedance state, which allow allows combining outputs to make a multiplexor. This could be designed into a discrete output circuit.

author: helloThere
author_rank: Contributor
timestamp: July 06, 2025, 02:09:55 pm
content: Do you have an example of what you mean by old school diode MUX? I see a few examples online, but I can't say I've designed a MUX before.

author: David Hess
author_rank: Super Contributor
timestamp: July 06, 2025, 05:07:46 pm
content: I will use the example shown below, which multiplexes three differential inputs into one differential output.The input signals and output signal are currents. The diodes for each signal are arranged in pairs, and an external control voltage drives one of the diodes of each pair, which causes the other diode of each pair to be reverse biased. The drive voltage must be able to supply more current than the maximum current from the input.In a high voltage application, the control voltage must also be high voltage. Performance is only limited by the current, voltage, and capacitance ratings of the diodes, so voltage, current, and frequency can all be very high.Operating with currents is not as difficult as it may seem because the output from the collector or drain of a transistor is naturally a current.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 06, 2025, 10:13:49 pm
content: Would reed relays be an option? Some automated multimeters use them to scan through a large number of inputs, they're rated for millions of cycles as long as they're not "hot switching".

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 06, 2025, 11:11:10 pm
content: Quote from: helloThere on July 06, 2025, 11:37:48 amFor what they do, those are not really expensive.Quote from: helloThere on July 06, 2025, 11:37:48 amThat would work, but you trade off PCB area for price.There are very cute SSRs in tiny package like Panasonic AQY221N5T, but they are lower voltage, targeting testers.These very-small parts are also not cheap, as they know people will pay a premium for the smaller size.Even in the jellybean parts, chasing smaller size costs more.

author: helloThere
author_rank: Contributor
timestamp: July 07, 2025, 01:31:40 am
content: My general mindset is at economies of scale, the price of components drops off quickly. That's why I'd rather go for a discrete component for a low cost version.I feel like board space is relatively cheap. I plan on making everything modular by using DIN41612 backplane connectors. I want to make this really low cost.