How to set a start and a stop digital trigger by conting the number of the trigger signals

[John_P1]

Hi Jiangjun,

Since they have a dedicated ADC per channel, simultaneous boards typically support only pseudodifferential or differential inputs and are specced as such.

    For example, the 6110 has 4 Pseudodifferential inputs (see specs page).  Pseudodifferential means that the (-) terminal does not handle AC signals as well as the (+) terminal.  A truly differential board has the same bandwidth on both terminals.

    As another example, the 6143 has 8 Differential inputs.

The 6251 (multiplexed board) on the other-hand has either 16 single-ended channels, or 8 differential channels (see specs page).

Best Regards,

[jzhj01]

Hi, John,

Thanks. And thank you all for all the kind help. It is really helpful to me.

Best,

Jiangjun

[jzhj01]

HI, Eric,

Just think of another important thing for my measurement: the dynamic range.

On NI's website, it says to be 20*log(Maximum/Minimum). So for a 14bit DAQ card, the dynamic range should be 20*log(2^13-1)=78dB. (It is 2^13 not 2^14 because of +/- values). Because The signal I am going to measure is just a positive one, if the maximum is 0dbm, the minimum signal is 30-40dbm. There is a 40dB range. So, the 14-bit DAQ should work very efficiently?...a 16bit DAQ can provide a dynamic range ~90dB?

Thanks a lot.

Best,

Jiangjun

[John_P1]

HI Jiangjun,

I'm assuming you're referring to this.

Anyway, looking at the code-width is not sufficient to determine dynamic range.  The article is simply showing the forumla to convert from Voltage to dB.  The minimum detectable voltage is given for some boards as a "sensitivity" spec, but this is can also be misleading since it requires dithering and averaging of the input signal.  The sensitivity spec is based on noise rather than on the code width.

What is the value of your signal in volts?  Our Multifunction DAQ boards are high-impedance--dBm is a power measurement so it depends on the resistance of what you are measuring across.  Also, many DAQ boards are only specced for DC accuracy rather than dynamic performance.

Best Regards,

[jzhj01]

Hi, John,

Thanks for your kind reply.

From the specs of our detectors, I think the input voltage can be several volt.

For example, the input power is 1mW, responsivity around 1A/W, and the output voltage/A is 5kV/A. So the output voltage can be 5V.

Consider a small input of -40dbm (0.00001mW), the output voltage is 0.5mV=500uV.

For this 14-bit device:

https://www.ni.com/en-us/support/model.pci-6133.html

Maximum Voltage Range       -10 V , 10 V

Maximum Voltage Range Accuracy       4660 µV

Maximum Voltage Range Sensitivity       432 µV

So, because the 0.5mV is too close to 432uV, it won't be accurately obtained. Right?

If I am using a 16-bit one with sensitivity of 132 µV, it can be definitely more accurate.

Best,

Jiangjun

[jzhj01]

Hi, John,

I am feeling confused from the beginning. It is about the 20*log(M/m). Why it is not 10**log(M/m)? Thanks.

Best,

Jiangjun

[John_P1]

A decibel is typically used as a representation of relative power.  However, voltage is proportional to the square root of power.

So, if you're talking about power, it would indeed be10*log(P1/P0).  To convert the formula to voltage:

        10*log(P1/P0) = 10*log(V1²/V0²) = 20*log(V1/V0)

Best Regards,