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

[slimfish675]

Hi shazam,

sorry if i bothered you with the circuit explanation of my last message. My intention was not to teach you but to show other people how the proposed circuit works. i’d like to emphasize that this series of posts are not because of my ego. I only want to contribute to enhance a product which can be very useful in my day work (when enhanced) and for many other people (like electronic enthusiasts).

The last circuit i posted had another error. This time was due to the drawing hurry (was done in 15 min). R5 and R? (i forgot to name it) are not grounded but connected to VREF. This is important, otherwise input can not cope with negative values.

The internal ADC in the ST uC (or any ADC) generally cannot resolve the number of bits claimed. The ENOB with noise factored in the calculations, and by empirical measurement, is lower by 2 or more bits (i.e. 14 bit → 12 bit, 16 bit ->14 bit, etc.) of reliable measurement. This impacts not only the S/N but the resolution out of a maximum 3 volts. The resolution determines the offset which can cause an error in the signal measured. Essentially, the offset numbers you calculated are much, much smaller the ADC can resolve.

Suppose the ADC can resolve 10 bit maximum with a VREF of 3V. That’s 2.9 mV/LSB. If offsets are in the range of 100uV, and we use a gain of 100 for the 10 mV/div, then we have a 10 mV offset which can be resolved by the ADC.

Note that most commercial scopes (Tektronix, Agilent, etc.) costing less than $3000 have only 8 bits of amplitude resolution (i.e. “Y” or “Vertical” resolution).

They cost thousands of bucks not only because the DC/AC resolution, but because of the sampling speed. And of course, for the functions they offer.

COUPLING
Perhaps I was not clear about AC / DC input coupling. I suggest ONLY AC input coupling for the reasons I mentioned. DC coupling for the full range of inputs would not easily fit within the DSONANO (the complexity of the attenuator, requirement for +/- supply, level shift to match the 3 V ADC, etc.). A simple1nF capacitor in series with the 10 or 100 kOhm input resistor would work down to a few HZ

Both presented circuits cope with AC and DC coupling for the full range of inputs. And i think they are not complicated at all (even easier to adjust). And as showed, you need no split(±) supplies, otherwise, the PGA113 would not fit the bill (5.5V max).

INPUT IMPEDANCE
The resistor I mentioned in the input is not to raise the input impedance, but to offer current limiting for the protection diodes in the PGA113. This is a very simple and inexpensive input protection circuit in case the probe is connected to a high voltage. The input impedance of the PGA113 is 10 GOhm (10,000 MOhm) with 5 pF capacitance. Changing the gain does not change this. Since most oscilloscope probes expect to connect to 1 MOhm , 5pF load, a 900 kOhm resistor (assuming a 100 kOhm input series resistor) is necessary from the input pins (2, 3) to ground. This is how the input impedance is defined. Note that there will be some small pF capacitors needed here to make sure the impedance is flat across the usable bandwidth. Most of the other PGAs have much, much lower input impedance and much, much higher capacitance.

If you get a 100kOhm resistor and use a 900kOhm in order to have a 1Mohm input then you have a resistive divider with a gain of 9/10. That’s fine when you have a signal with an amplitude smaller than 10/9 of VCC, which is not the case of DSO Nano, capable of -+40V with a x1 probe.

BANDWIDTH
As for the GBW of the amplifiers, note that they all vary with the gain of the PGAs. The PGA113 can have a gain of 50 and still exceed the DSANANO bandwidth. I guess if you want to measure micro volts, than this will not be sufficient. I can’t see using the DSONANO for use with very small signals because of the total lack of shielding, the input connector, and the probe arrangement. The DSONANO can be useful from a few HZ to perhaps 499 kHz with input signals greater than 10 mV. With 10 mV input signals the accuracy will not be very useful, but signal shape can be analyzed. To measure signals with reasonable accuracy, the input should be greater than 50-100 mV.

Again, in order to have a ±40V input with a x1 probe it’s necessary to atenuate the signal first. If you connect a x10 probe to the input, then signals will be very small and they have to be amplified (every mV is important).

OFFSET
As for offset, refer to U5A in the DSONANO. The sole function of this circuit is to DC level shift the signal. The input to this circuit is via R15. The uC outputs a PWM here, which is integrated by R15, C12 and R16, C13 into a variable (depending on PWM) DC voltage to pin 3 of U5A. U5A functions as a buffer changing the DC level of the input signal from U5B before it goes to the uC ADC. If you replace U5 with a PGA, then the PGA has to have a variable offset input, which is pin 4 Vref on the PGA113. This is necessary even if the input is DC coupled to compensate for any offset and is probably also used for vertical positioning of the waveform on the LCD display.

There is no need to have a variable offset input when you can select between AC or DC coupling. The original scope doesn’t have a real AC coupling and the offset “simulates” and AC coupling by adjusting with the PWM the DC component. The PGA113 amplifies the signal in relation with VREF making the AC “simulation” a challenge programming effort in the case of this PGA.

10 CHANNEL INPUT (PGA117)
This was in response to a few posters who asked about the possibility. You are correct that this would not make the DSONANO a 500 MHz Logic Analyzer, but limited to perhaps 100 to 300 kBs. I think that this would not be very useful because of the small screen and low resolution the DSONANO. But if there are people interested in this, it is a possibility. There are USB 8+ channel Logic Analyzer probes with similar performance for about the same price already, and they show the multiple waveforms on the large PC / Laptop / Netbook screen.

You’re right, for two channel, up to 300 ksps are possible. But samples are not taken at the same time, so the aliasing probability is very high.

AUTO OFF
Perhaps what you need is an external battery pack. Model RC airplanes and helicopters have readily available LiPo batteries about the same size as the DSONANO which could operate it for 20-40 hours continuously. Of course this would more than double the thickness and weight of the DSONANO, cost $20-$40 and require a separate charger $20-$20.

Thank you but no. What i need is a self contained oscilloscope for field use. And it’s not complicated to have an oscilloscope which enter low power mode if battery gets depleted (PGA113 has a software shutdown mode).

SCHEMATIC from Slimfish of 6-22-2010

A few changes suggested:
• AC Couple both channels.
• Delete the 1 MOhm from IC pin 2. This also allows using a 1 nF or 0.1 nF COG (do not use X7R, X5R, Y5U, etc.) on pin2. → You’re right with the previous schematic. But this resistor is not connected to GND (see new schematic) and thus is needed anyway.
• R1, R2, C7 need to be 805 or 1206 for >200V capability. C7 and C? (647pF to GND) need to be COG types
• Delete C8, the ADC has enough capacitance. If a low pass filter is desired here, increase the value of the 100 Ohm resistor and use the input capacitance of the ADC to calculate R. → The ADC input capacitance is about 5 pF. It’s very dangerous to count on such small (and variable) values to conform a low pass filter. But as showed on current DSO Nano it works…
• Delete the entire Vref section (OpAmp and related parts). Connect pin 4 of the PGA113 to the same place as U5A pin 3 (R13 - C16 junction) in the current DSONANO. → No, you cant. VREF is a low impedance one (3.25 kOhm -see datasheet-) and hence the PWM filter will not function as intended.

Slimfish