DSO NANO - how we users can impact next hardware design !!!

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slimfish675
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Re: DSO NANO - how we users can impact next hardware design !!!

Post by slimfish675 » Tue Jun 01, 2010 1:43 pm

Hi Shazan,

i'll try to explain to you although i have only a couple of minutes right now.

When circuit is USB powered, no problem at all.

If circuit is battery powered, as there is some current consumption, there will be a voltage drop across transistor. Supposing transistor has 100 mOhm and circuit draws 200 mA => 200mV. If battery is only a bit discharged (say 3.5V) then the regulator (3.0V) is operating in its limits of regulation (not really, but near).

The SD card isn't the problem here (many of them work in the 2.7V-3.6V range) nor the Cortex (2V-3.6V) but the ADC reference. ADC uses VDDA to reference the VREF+ so if battery voltage drops say 3.3V (as the current in the VDDA branch is lower), we are losing accuracy in the DSO readings.

Of course, i consider this a trade off and otherwise the circuit looks very good to me.

Slimfish

Note1: i agree that the SP3232 trick is a neat one, but the output regulation is far from ideal (no linear regulator inside, it uses a discontinous mode with a 5.5V treshold).

shazam
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Re: DSO NANO - how we users can impact next hardware design !!!

Post by shazam » Tue Jun 01, 2010 4:04 pm

Slimfish,

E=I*R, therefore: 0.200 Amps * 0.100 Ohms= 0.020 Volts. The Fairchild data sheet shows that with a gate voltage of -3.5 V the Rds is 0.100 Ohms.

Shazam

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Re: DSO NANO - how we users can impact next hardware design !!!

Post by slimfish675 » Tue Jun 01, 2010 4:14 pm

Shazam,

you are absolutely right. I unfortunately shift the units.

Slimfish

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Re: DSO NANO - how we users can impact next hardware design !!!

Post by slimfish675 » Tue Jun 01, 2010 5:00 pm

Shazam,

I started to draw the schematic in EAGLE format. I have modified a little your original schematic because i thought R21 (1Ohm) and the 10K resistor are not needed. Maybe should we keep R21 for current limiting purposes in the switching process, but i think thats not an issue.

Circuit diagram is in the image below. It's also necessary to reduce the charging current a little in order to reserve some of the USB current to power the DSO Nano.

The MCP73831 has a pin to indicate if battery charging is complete. So maybe it can be used to report charging status in screen (only one digital pin needed).

Slimfish
Attachments
Battery - EAGLE 5.7.zip
EAGLE Schematics
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Battery_Charger_A.GIF (16.85 KiB) Viewed 11281 times

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Re: DSO NANO - how we users can impact next hardware design !!!

Post by shazam » Tue Jun 01, 2010 11:42 pm

Hi Slimfish,

Your schematic looks good.

R21 was added by Seeedstudio in Rev 1.3 of the PCB. It looks like a current limiting device, but its real function is a fuse. Since there is no charging IC, if the battery & diode shorts, the power dissipation in the 1 Ohm resistor is nearly 100 x the rating of this 402 device, so it will open.

You are right, this resistor is not necessary with a functioning charge control IC.

The 10 K resistor I added is not necessary, since R25 and R26 will discharge C16 and turn on the FET.

I never looked at the charge current with the IC. Since you brought it up, and other postings have mentioned issues charging from USB ports, the charging current is way too high. Since we don’t have access to the battery manufacturer’s specifications, and the circuit does not have thermal sensing of the battery, fast charging at 1x the battery Capacity (C) is another way to shorten the life of the battery.

Normally the standard charge rate for these batteries is C/10, which in this case would be 0.5 Ah/10= 50 mA per hour. This means a fully discharged battery would take 10 hours to charge. Without more information about the battery, I suggest a compromise of 100 mA charge current, yielding a full charge in 5 hours. This would require R27 = 10,000 (10k) Ohms.

Connecting the “charge state” pin to the uC should be straight-forward. The IC has a version with a 3 state output indicating “charging” and “charging finished” (MCP73831, same cost).

There are excellent Battery & Power Management ICs from many vendors which could be used, but they cost significantly more (>2-5x).

NOTE to all in this post: two of the outstanding features of the DSO Nano which attracted me are the price and size. While firmware features and PCB traces have no production costs, hardware parts do. If portable full featured, 2 channel DS scope is what you want, they are available for $500 - $600 (i.e. Hantek and Owon). Many of the choices made by SeeedStudio is this design were obviously made with getting the most performance for the least price. So while we can suggest improvements, try to keep in mind these factors.

Shazam

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Re: DSO NANO - how we users can impact next hardware design !!!

Post by slimfish675 » Wed Jun 02, 2010 12:33 am

Shazam,

as i bought DSO a few months ago maybe the capacity has increased but the battery my DSO has is 680 mA (at least in the markings). Although standard charging rate is safer for the batteries, i think something slightly greater than the standard charging rate can be used (say C/4 or C/3). Charging it in three or four hours seems a good battery life/usability constraint.

I agree with you in the fact that DSO is attractive because is small and cheap. But i also think that hardware can be enhanced for a few (<5) bucks more. And that's where the funny game starts.

I also think that this circuit in particular can`t be improved much more. So maybe it's time for others to comment on it (if they found any flaw) and to proceed with other DSO blocks.

My next proposals:
-Power management (simple). If you forget the switch on -> battery dead. This is a mix of hardware and software, but if there is no hardware to support it, software will never "use" it.
-Dual channel input (complex). Duplicating the hardware is the obvious solution. I want to make a better aproximation. This point could try to address
*Variable input impedance of actual stage (1 MOhm is only in the manual)
*AD+DC measurements. Actually the harware support some kind of AC measurement (thanks to PWM feedback) but as far as i know, it has been never implemented in software.
*Noise
*Automatic calibration
-Signal output (medium)

Enough for today. By the way Seeedstudio... it will be very interesting to have some feedback from you!!!

Thank you very much,

Slimfish

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Re: DSO NANO - how we users can impact next hardware design !!!

Post by esp » Wed Jun 02, 2010 11:52 am

Thanks Shazam and Slimfish, I'm a bit dazzled about your conversation. Seeed didn't design the circuits, you may see from the box that it's the work of Mr. Cai Xiaoguang (aka Bure if you read the source code), a very veteran electrical engineer and luckily our closest partner. We are more apprentices in this product, I'm consolidating all available resources from inside and outside Seeed Studio to make DSO nano better.

I'm keeping consolidating the ideas from forum, blog ,email or anywhere I can see them. We will add the charging circuit in next revision soon, along with other improvements <5$ for current version. I will generate a list of fix/improvement/TBD soon.

For SP3232, we will modify it in the next version with TC1240.

Thank you so much for the suggestions! I will get back to you soon.

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Re: DSO NANO - how we users can impact next hardware design !!!

Post by shazam » Wed Jun 02, 2010 2:42 pm

Hi ESP,

I am working on a better input amplifier already. It is busy where I work, so it may take several nights. My goal is to also eliminate the '4051 switch. I am also thinking about equalization for the X1 and X10 probes.

If you are thinking of using the TC1240, I suggest looking at the TI TPS60403, which is almost 1/2 the cost.

This function (power supply for the input amplifier) should be chosen to match the requirements circuits (ICs) of the input amplifier.

I will keep you updated with what I find for a better input amplifier circuit soon.

Shazam

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Re: DSO NANO - how we users can impact next hardware design !!!

Post by slimfish675 » Wed Jun 09, 2010 11:17 pm

Hi all,

in my last message, i proposed a few DSO areas that could be improved. As i think input stage was the most difficult (and interesting one) of the list, i started with that.

My basic design goals:
-Cheap enough (<$5)
-Input impedance 1 MOhm for the whole input range (x10 probe friendly)
-AC+DC capable
-More accurate & less noisy
-Easy to calibrate (Offset + gain)

Other less important goals:
-Small size
-Single supply (while conserving ground for reference)
-Easily extendable to dual channel

The image below shows the designed circuit. As depicted is dual-channel, but can be easily ported to single channel. The integrated circuits used don't have to be those exactly. Input stage AD8615 (single op-amp) can be replaced by a cheaper dual op-amp version (AD8616) or by less capable version (like AD8602). Both op-amps are from Analog Devices, but any other that fits input noise, input current, Vos & GBW parameters can also be valid.

For the programmable gain amplifier a LTC6912 is used. It's digitally controllable, dual channel, low noise and has an integrated midsupply. Other alternatives exist and can also be used with minor modifications/additions (MAX9939, PGA113, AD8231...).

In terms of cost, TL082, SP3232, HC51 are not needed anymore so althought initial IC cost is near the $5 for the dual channel version, the final cost will be lower.

As i said in a previous message, this schematic is only a schematic (a very first draft of it). Althought i made some minor simulation of it, it's necessary to build it to check if everything works as expected. I expect to build it in a few days to validate it. In the meantime, all comments/suggestions/complaints are welcome.

Slimfish
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Dual channel DSO input
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Re: DSO NANO - how we users can impact next hardware design !!!

Post by shazam » Sat Jun 19, 2010 11:46 pm

Hi Slimfish and all,

I have finally had some time to spend on the Programmable Gain Input stage. Slimfish, the PGA ICs you list and the LTC you feature in the schematic all have different characteristics. The LTC6912 has poor input characteristics (low impedance) so it requires the two op amp buffers you added. Another issue is there is No way to adjust the offset on a large scale. This is likely used to adjust the vertical position of the trace by the U10 pin PB11 ST uC.

After having evaluated the parts you list and many more, the TI PGA113 will provide the High Input Impedance, Low Capacitance, Low Offset & Drift, Self Calibration, and a Reference Input which can be used to move the trace vertically. It also has two separate power supply pins, one for the amplifier and one for the output buffer to match the ADC in the ST uC. The PGA113 has a two channel multiplexer, a high constant impedance and the bandwidth, noise, distortion and drift are more than adequate for the DSO Nano.

It runs off the existing +5 and +3 V power supplies (does not require an +/- supply) and does not need a separate input buffer. So this eliminates the U4, U5, and U7 ICs. Note that U7 also is the output buffer for the test signal (U10 pin PD 12), so a small buffer op amp will be needed.

To use this part, close attention will be needed to properly de-couple the power supplies, filter the PWM out from U10 pin PB11 for a clean DC offset voltage (may also require a Buffer Op Amp), and layout and shielding to minimize noise. The inputs should have a 10,000 Ohm or higher resistor in series to limit overload. A 100 kOhm HV resistor would be better (protects to 1000V input) but it may affect the max bandwidth a bit.

The part is less than $2.00 (about $1.00 in a full reel quantity).

Another possibility is to use the TI PGA117, which has 10 multiplexed inputs. This would add the ability to use it as a 2 channel analog input with an additional 8 channels of logic. Unfortunately this would require something like a 10 pin 1 mm header and a cut out in the case to access the connector for the 8 Logic Inputs. I suspect that the uC would require additional memory (external) as well. Just a thought.

Auto Power Off and DC Input. I think most people would not want a simple Auto Power Off, unless it could be adjusted for time and disabled. This may cost more than it is worth. With a good charging circuit, battery life should be good, and when connected to external USB power, it is not an issue. Small USB power supplies are readily available for a few dollars on e-bay and elsewhere. DC input would be difficult to implement, especially with some Overload Protection for the DSO Nano.

Shazam

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