ADC Sigma-Delta Range and AC signals..

[Anonymous]

Dear community,

I've been working with ADC sigma delta in resolution of 18 bits. I need to rest 2 signals (differential signals), like 2-5mV ac signals. Despite they're very small I need to put them over an offset, in this case of 1.024V available from psoc, 'cos I want to preserve low level of noise as  suggestions observed in AN84783 page 11 : Input ± Vref to have the lowest noise and I can't bypass the buffer 'cos I have observed offset falls to 0.980V but even when I change to rail to rail or level shifter there are no changes; offset still falls.

In page 9 of reference document of ADC Delta-Sigma 3.30 the range with Input ± Vref  with my Vref=1.024V on offset also should be: -1.024±1.024= 0V to 2.048. With all of these considerations I have the next questions:

1.- Why in example of CE95271 with the same condition as I, in page 2 says that if positive enter of ADC is 0 the converted value is -1.024? or if is 1.024 the value is 0? so the range doesn't go from 0V to 2.048 as page 9 of ADC Delta-Sigma 3.30? Please clarify this I'm very confused.

2.-. I have read there's a example of how to manage negative voltages on psoc, blog that I couldn't find unfortunately, but is related on applying offset, in this case 1.024 as AGND. Could you tell me where I can find this:  http://www.cypress.com/blog/psoc-hacker-blog/measuring-negative-voltage-using-psoc1????

3.- Is it possible to interface directly these signals without offset moving input reference as low as Input ± 0.0625 * Vref?

4.- Do you suggest is better implement an instrumental amplifier with pga's and opamps instead of using ADC Delta-sigma? or in worst case an external INA?

I hope you can help me with your promptly response, thanks in advance.

[odissey1]

Too many questions! It is better to describe specs requirements (voltage, offset, frequency, resolution, impedance, etc) and a source of the signal (with schematic).

2. For Handling negative voltage on PSoC see this thread and links within:

I need an ADC can take Negative voltage

and this

PSoC 6 MCU Current Digital to Analog Converter (IDAC)

4. DelSig-ADC has differential input +buffer amplifier. You don't need Instrumentation Amp made from internal Opamps - it is worse than DelSig native differential input. 2-5mV can be handled by PSoC (but the input signals must be in specified range).

/odissey1

[Anonymous]

Thanks for your response,

Do you think ADC Sigma-Delta is capable to manage signals as low as 5mV p-p without external circuity?

Thanks for your time and suggestions.

[EvPa_264126]

ADC Sigma-Delta is capable to measure AC signals as low as 200 mV p-p without external circuitry.

it for Buffer mode = Level Shift or Bypass Buffer (I got even up to 400mV p-p)

and you can increase the buffer gain.

[Anonymous]

Eveniy, Thanks for your response, but you need level shifters to work, don't you? 'cos if signal goes is ±200mV p-p PSOC5LP can' handle more than -200mV, this could damage the device. In fact you have a response for this:

https://community.cypress.com/message/155563#155563

Tell me about level shifters, thanks in advance.

[EvPa_264126]

I do not use a  level shifters if the signal is less than ± 100 mV. The input of the ADC has protective diodes.

Therefore, it is sufficient to install a resistor at the input to to protect ADC against large input signal.

[odissey1]

I tried to apply negative voltages to PSoC inputs, and it gets clipped at about -0.7V by protection diodes, no damage to PSoC observed. Obviously, there should be a resistor (~1k is enough) placed at such input to protect the diode from overrcurrent.

/odissey1

P.S. What frequency range for AC signal you are looking for? or it is a DC signal 5mV?

[Anonymous]

Thanks for your response. It's a signal from ECG sensor, which I need to mount over a CD level. That's way my doubts.

[EvPa_264126 · Accepted solution]

Look at  Ad8232

At this forum there are several discussions about the ECG

my old projects for PSoC5

[DeborahRich]

At this forum, there exist several conferences around the ECG