Question
I have some doubts for the datasheet of AD2S1210:Q1:In the spec of AD2S1210, we can see the RDC system response block diagram in Page 32. There are five parameters:K1,K2,a,b and c. (Figure 1)They are the parameter for integrator transfer functions and the compensation filter transfer function. We can get the open loop transfer function ofthe R( Figure 2)So G(z) should include the parameters of K1,K2,a,b and c, but it is strange that in the formula of the finial transfer function G(s), the parameter“c” disappeared. Please refer to Figure 3Are there some problems from G(z) to G(s)?Q2: There are two Table of RDC parameters,Table 28 and Table 29 (figure 4 and Figure 5):The relationship of the parameters in Table 29(t1,t2 and Ka) and that in Table 28 is as below (Figure 6):But if you try to calculate t2 and Ka according to the formula above with the parameters in Table 28, you can find the results are not equal to thenumbers in Table 29, they have a big difference!For example, if 10bitt2 = t(1+b)/2(1-b) = 244ns * (1+509/512) /2/(1-509/512) = 4.152 ns = 4.152e-3 ms, it is not equal to the “t2” in Table 29 which is 42 ms! Thedifference is 10000 multiples!The same problem with Ka!Could you please explain the reason?
Answer
I’ve gone through the equations and got some more info from the design team.There is a typo on Table 29. t2 should be in us as you pointed out.And c^2 is also an element of Ka when going through the formulas. T the sampling period is also a factor .
The attached excel sheet calculates the values for Ka, t1 and t2.Please also note that the k1 value to calculate Ka is 0.63. This is derived from (3.15/2)/2.5 which is half the typical input voltage not the 1.8/2.5as outlined in table 28.
I hope this clarifies the point.