I'm using the pga112, but I have no idea about how to choose RA value about PGA112. Here it's the detail:
My applicationn is similar to Fig 77 of PGA112 datasheet. I have two input signals, they are both bipolar signals, about +/-100mv, just like the Vin0 in Fig 77, the difference to Fig 77 is my Vin1 is also bipolar. I'm trying to copy the Vin0-Ch0 application to Vin1-Ch1 application. Like this:
Vin0(bipolar +/-100mv)---Ca0---CH0,and Ca0 also connect to RA0, Ca0-RA0-VREF(PGA112）
Vin1(bipolar +/-100mv)---Ca1---CH1,and Ca1 also connect to RA1, Ca1-RA1-VREF(PGA112)
(Two Channels, so there are two Ca,RA).
Here it's my questions:
1:) How to Choose RA Value?
In Fig 77, the datasheeet gives guideline on how to choose RA values, and you can see the RA value is determined by Vin1-Ch1, it has nothing to do with Vin0-Ch0.As my application is different with Fig 77, then how can I choose RA value?
In Fig 77, you can also find the Ch0 is about 2.5+/-0.1V, VREF is 2.5V,it means there is no voltage drop on RA, does it mean RA is equal to 0 ohm?
2:)Will there be disturblence between Ch0 and Ch1?
As RA0 and RA1 both connect to VREF(PGA112), do CH0 and CH1 disturb each other?
Per my application, is there any other way that I can use PGA except the one I mentioned above?
My application is two bipolar input and I hope the pga output signal is 1.65V centered, Pga112 output is connect to TMS320F28027.
Figure 77 in the PGA112 datasheet shows two different approaches to scale a bipolar input signal to a useable voltage range for the PGA112, Vin0 is AC-coupled through the capacitor while Vin1 is DC-coupled. In the AC coupled application (Vin0) the capacitor and the resistor RA form a high-pass filter with a corner frequency at 1/ (2*pi*R*C) therefore the value of RA is determined by the necessary low-frequency response and the value of the input capacitor. As you noticed in the figure, there is no DC voltage drop across resistor RA, this is because the input impedance of the PGA112 is very high (10 Gigaohms typ) and so very little current flows through this resistor, producing negligible voltage drop. This DOES NOT mean that RA is 0 ohm, in fact making RA a small resistance will create a very high corner frequency as shown in the previous equation.
As for your second question, the amount of disturbance between the two channels depends on the output impedance of your voltage reference (should be very low) as well as the common-mode rejection ratio of the input amplifier (usually very high). Because a good DC voltage reference will have a very low output impedance I would not expect much disturbance between the channels however, as frequency increases inter-channel crosstalk will increase.
Analog Applications Engineer
PA Linear Apps
Thank you very much for your answer!
This issue have confused me for more than a month and now I understand how to choose Ra. For the disturbance, I'll follow your instructions and try again, I may add a jump between input1 Vin1 and Vref(PGA), if it disturb too much, I'll break it.
Next edition will be ready in 4-8wks, I'll update the result then.
By the way, I find your suggestion is very helpful so I tried to connect "submit documentation feedback" on the bottom of pga datasheet, but I failed to open the web. I hope your suggestion appeared in pga datasheet so the other reader can understand the AC-Couple easily, furthermore, I guess there are written mistake about Ra,Rb in Fig78. Another suggestion is name Ra in Ch0 Ry or something else, as it's not necessary be same with Ra in Ch1.
Today I had my new edition of analog board ready, let's name it A3.
Here it's the result: PGA112 in A3 doesn't work properly. The gain can not be changed, it's always be gain==1;the channel can not be changed, it's always be Channel==Ch0;
I use C2000 TMS320F28027PT to drive PGA112, and the 28027 also drive pga112 in old edition, let's say it's A2 well. Both the gain and channel can be selected, with the same program, the 28027 can not drive PGA112 in A3 well.
The 28027 program is modified from TI Example(V126, spi-loopback).
Here it's today's test procedure:
In early of February, similar situation happened with A2, at that time, gain and channel are also can not be changged, then I took off pga112, channged with a new pga112, it works well.
Here is my question:
Please comments at your convience.
It would be very helpful to have a schematic of your circuit employing the PGA112 in order for me to determine what is happening. To clarify your message above, it sounds as though the PGA112 that is installed in board A3 is being damaged because swapping the part into board A2 does not restore functionality of the part. Is this correct?
A3: PGA112 and Upper side
J8,J9 is connector to 28027.
About your clarify: You are right, PGA112 in A3 board could be demaged.
My signal is 40KHz, so C13=0.01uF and R29+R30=532ohm, the pass frequency =29.9kHz
It looks like:
I ordered new PGA112 and will have another A3 board made.
Thank you for your help very much and look forward to your comments!
It looks like you can not find my schematic. Actually I posted my schematic and I found it in the confirm email e2e writing to me. Anyway, the schematic is not so important now as the pga works.
It looks like the problem is with "shut down" mode. As PGA112 has no hardware shutdown and I have not set shutdown mode, so I did not think about it. One Chinese TI support engineer remind me my problem is similar to shut down mode. After check all hardware issue, I tried to write E100H to disable shut down mode, then write the SPI command data sucessfully, the pga will work well and badly in a few cycles, after that it works well.
This afternoon, I found the pga works well sometimes and doesn't work sometimes, then I found it's necessary to give pga enough time to receive spi data as spi is in low speed, the best way is have my 28027 null loops for some time after SPI data writing.
Your suggestion on Ra selection is right! The signal voltage is centered in 1.65V now leave alone the gain.
Thanks for your advice!
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