# Measuring Voltage up to 4 decimal places using TDS3032 Oscilloscope

#### Tahsina Hossain

Joined Jan 16, 2015
40
Hi Everybody,

I am trying to capture waveform data from TDS 3032 oscilloscope through RS232 port. All samples have 1 decimal place data only (i.e. 20.3V) whereas I need something like 20.2985V. Is there any way to get 4 decimal places from oscilloscope?

I really appreciate any help.

Tahsina

#### MrChips

Joined Oct 2, 2009
19,906
No way. Most digital scopes are 8 or 9-bit resolution.
The TDS3032 is 9-bit vertical resolution, that is 1 part in 500 resolution.

Do the math. Four decimal places such as 20.2985 is a resolution of 1 part in 200,000.
You would need an 18-bit ADC and less than 100μV noise.

Last edited:
• Tahsina Hossain

#### crutschow

Joined Mar 14, 2008
24,035
To get higher resolution you would need a fast, high resolution digitizer (A/D converter). But you likely won't be able to reach 18 bits.
What is the highest frequency of interest in your signal?

#### Tahsina Hossain

Joined Jan 16, 2015
40
No way. Most digital scopes are 8 or 9-bit resolution.
The TDS3032 is 9-bit vertical resolution, that is 1 part in 500 resolution.

Do the math. Four decimal places such as 20.2985 is a resolution of 1 part in 200,000.
You would need an 18-bit ADC and less than 100μV noise.
Thanks a lot for your reply. Can you suggest any oscilloscope that can do this?

#### Tahsina Hossain

Joined Jan 16, 2015
40
To get higher resolution you would need a fast, high resolution digitizer (A/D converter). But you likely won't be able to reach 18 bits.
What is the highest frequency of interest in your signal?
Well, the voltage signal is 50Hz. Is this what you are asking?

#### crutschow

Joined Mar 14, 2008
24,035
Well, the voltage signal is 50Hz. Is this what you are asking?
Yes. That eases the problem somewhat.
So why do you want such high resolution for a 50Hz signal (I presume this is the main's voltage)?

#### MrChips

Joined Oct 2, 2009
19,906
I am not aware of any oscilloscope with that kind of resolution.
You need to find a DAQ system. Search google for high resolution DAQ.

#### Tahsina Hossain

Joined Jan 16, 2015
40
Yes. That eases the problem somewhat.
So why do you want such high resolution for a 50Hz signal (I presume this is the main's voltage)?
Basically I have a sense resistor (0.156ohm) at an inverter output. I am using two probes to acquire voltage level before and after the sense resistor so that I can deduct one from another to get the voltage drop across the sense resistor. The objective is to find inverter output current. However, since I am getting only one decimal place, the two probes are giving something like 19.5V and 19.4V but this is not giving me proper voltage drop. According to my theoretical calculation, the inverter output current is 62mA, which means there should be 0.009672V drop across the sense resistor. I hope I could explain my problem to you.

#### MrChips

Joined Oct 2, 2009
19,906
Leave the two probes where they are and select the MATH SUB feature of the oscilloscope.

#### Tahsina Hossain

Joined Jan 16, 2015
40
Leave the two probes where they are and select the MATH SUB feature of the oscilloscope.
That is the obvious thing to do, right? So I tried that a lot of times. The probe voltages are almost similar in amplitude (around 20V) which means the result of MATH function is a very low voltage (milli-volts). So when I change the vertical volts/division to milli-volts level, say 100 mV/division what I see in the scope is almost like a noise rather than expected outcome. I captured the data (which seemed like noise) and observed manually that the data values are not consistent, means its noise plus data as the data is at milli-volts level. What can I do?

#### MrChips

Joined Oct 2, 2009
19,906
Have you tried using a DMM to measure the voltage across the resistor?

#### crutschow

Joined Mar 14, 2008
24,035
You can't change the scale to mV levels when using the math function since that causes the the scope input amplifiers to saturate.
You need a differential amplifier with good common mode rejection at a high voltage to measure that voltage. TI, among others, sells such amps.

#### Tahsina Hossain

Joined Jan 16, 2015
40
Hmm, seems like I am out of options at hand at the moment :-(

#### crutschow

Joined Mar 14, 2008
24,035

#### Tahsina Hossain

Joined Jan 16, 2015
40
I have fluke 39 but it cant measure current below 1A, my output current should be around 62 mA

#### DickCappels

Joined Aug 21, 2008
6,068
You might want to use a high accuracy DMM to measure the voltage across the resistor.

#### tcmtech

Joined Nov 4, 2013
2,868
You could just change the resistor out to a larger value one and increase the voltage drop across it or you could do a direct millivolt reading across the resistor and calculate your current from that.