Zmod Scope Alternate Input

[Mark K]

The Zmod Scope has an input range of +- 1V (high gain) or +- 25V (Low Gain).  To match the +-5V high range output of the Zmod AWG, is there a know set of resistors/capacitors that can change one of the Zmod Scope inputs range to +- 5V?  Is there a way to order the Zmod Scope with alternative input ranges?

[artvvb]

Hi @Mark K

Unfortunately, Digilent doesn't offer any variants of the Scope with different input ranges. Is the resolution loss from using the Scope high gain mode significant for your application?

Thanks,

Arthur

[Mark K]

@artvvb

We are looking at a signal that is typically 0 to 3V or -1.5 to 1.5V with the ability to measure the DC offset being important.  We would like to have a higher fidelity of the signal as we are currently wasting most of the input range.

I have been considering changing the gain resistor dividers, however many of the components are missing silkscreen and I have not found an assembly drawing for the PCB.

[zygot]

On 11/7/2022 at 9:39 AM, Mark K said:

We are looking at a signal that is typically 0 to 3V or -1.5 to 1.5V with the ability to measure the DC offset being important.  We would like to have a higher fidelity of the signal as we are currently wasting most of the input range.

Your post is a good illustration of an inherent problem with converting analog signals into the digital domain. It's really not possible to design one piece of hardware to accommodate every possible application. As a general purpose ADC converter the ZMOD ADC pods are very good, but not idea for everyone's project needs. Even if they were designed to have a fine-grained user adjustable input gain front end to meet one user's particular requirements it would introduce other problems and make using them much more difficult. I idea that one can connect a generic ADC circuit to any analog signal and not have to work through an analysis of the performance of the ADC circuit is a poor one.

Analog Devices has some good basic information available that covers this topic for people who want to use ADC converters instead fo designing them. Unfortunately, in most cases, if you want to use an ADC you need to do some design analysis in order to use one.

You can always design an analog front end that scales your analog signal of interest to the ZMOD input range. This will maximize the dynamic range of your converter output codes. It will also force you to consider what your front-end plus the ZMOD front-end is doing to your signal. While this doesn't sound very appealing it's the only correct way to go. It unfortunate, but nothing that you can do to fix one issue with your analog front-end comes without negative consequences.

In the real world ADC circuits, consisting of a converter and an analog front-end conditioning circuit, are highly application specific and often require some sort of AGC or PDA to accommodate a wide range of signal levels. There's a lot to consider besides voltage range. Just as important are input impedance, frequency response, etc. Once you've worked out all of those details and have digital data you aren't done. If you expect to be getting 15-bit digital dynamic range then you need to work out the ENOB for not only the converter but the combination of the converter and the associated front-end conditioning circuitry. If you are hoping that there's such a thing as zero offset for your digital data then there's more analysis to do before using that data.

The good news is that, if you know what to expect in terms of what your ADC is doing to your signal it's possible to mitigate undesirable effect on your data with DSP post processing.

Edited November 11, 2022 by zygot