11-29-2011 07:03 PM
Hi, I'm using a Spartan 3e kit boart. The ADC part on the board is consisting of a AMP and ad. From the UG230, the input analog signal must be in the range of (0.4v~ 2.9 v)? The peak-to-peak value of the signal I want to send it into the board is around 0.2 v. So what should I do? Another a circuit to amplify it? THX
11-29-2011 08:32 PM - edited 11-29-2011 09:02 PM
Therefore, to avoid clipping, the gain supplied by the programmable gain amplifier must be no greater than
---- = 12.5
The nearest gain setting value is -10 (UG320 Table 10-2).
Your 0.2V P-P input signal should be AC-coupled to the programmable gain amplifier, with a DC bias to REF voltage developed by divider R132+R129 (schematic sheet 11 of 14).
Using Equation 10-1, the resulting range of input values from the ADC should be
VIN - 1.65V ± 0.1V
D[13:0] = GAIN x ----------- x 8192 = [-10] x -------- x 8192 =~ ± 6553(d)
The ADC output (14-bit 2s complement) value should range from 0x2667 to 0x1999, or ±1999(h).
This topic belongs in the Xilinx Boards and Kits forum. The Spartan Family FPGAs forum is for discussing technology and implementation details which are specifically related to the Spartan devices and packages.
-- Bob Elkind
11-29-2011 09:43 PM
Thank U very much! I'm searching and reading the datasheets of these three, 2624, LTC 6912-1 and LTC 1407. So should I use an extra power supply to provide a 1.65 V to the J7 header's REFAB? I measured the VCC and GND and found out its voltage is 3.3 v. Does that mean there's no need that I add a VCC to the J7?
11-29-2011 09:55 PM
So should I use an extra power supply to provide a 1.65 V to the J7 header's REFAB? I measured the VCC and GND and found out its voltage is 3.3 v. Does that mean there's no need that I add a VCC to the J7?
You haven't explained why you are asking these questions, so I'm unsure how to answer.
-- Bob Elkind
11-29-2011 11:49 PM
Hi, I want to make the ADC part work for the signal which is obtained from a sensor. The p-to-p voltage is around 0.2 v, and according to your reply, I know the gain setting of that pre-amp on the board. But what I don't know, these 6 pins in J7 on the kit board, REFAB, REFCD,VINA, VINB, VCC, GND , what should they be connected to ?
11-30-2011 12:26 AM - edited 07-12-2012 01:35 PM
J7 pin/signal connection
VINA sensor output (AC-coupled?)
GND sensor GND
Remember that your sensor output signal must be centred around the REF input to the programmable gain amplifier. The easiest way to ensure this is to AC-couple the sensor output (to J7 pin), and add a high-value R (as a DC bias) from the REF voltage to the PGA side of the input cap used for AC coupling.
Remember: AC-coupling implies a high-pass filter applied to the output of your sensor. Is the sensor output a DC signal or an AC signal?
Does this make sense?
-- Bob Elkind
07-12-2012 01:03 PM
I try to use Spartan 3e kit borad to captuer a sine signal, but when I add some bias to the signal, the analog value changes. For example, the input is y=0.4+sin（x), which means the peak value should be 1.4 volt. However, when I connected this signal to VINA on the board and turn on the board, the peak value reaches to 1.7 volt. I set the gain as -1 and all values have been observed with scope.
By the way, if I give DC value or sine signal, it works very well, so I don't think there is any problem for my code. Is that because I mix AC and DC? However, I must need them mixed for my project.
07-12-2012 01:48 PM - edited 07-12-2012 01:51 PM
Welcome to these forums. Please take the time to read the New Users Forum README thread. There you will find the following:
4. Please do not post a new topic or question on someone else's thread -- start a new thread!
Interrupting a discussion with an unrelated topic or question is not simply rude, it is considered disrespectful. This is called hijacking.
Please do not request assistance by way of private messages unless your problem requires privacy or offers fantastic exchanges of wealth.
Finally, if your input voltage is too high, you should consider reducing it. If this is not possible, then consider reducing the input amplifier gain.
-- Bob Elkind