Digipot - linear (need log) - can charactersitic be changed in code?

[HankMcSpank]

I've a circuit where I need to use a dual digipot - the thing is I need one to be linear & one to be log - they don't make up hybrid - it's one or the other. I really don't have PCB space for two separate pots, so just wondering is there anyway this can be spoofed in code?

I'm using a linear pot as input...I'm thinking here along the lines of some clever maths on the AtoD sample (ie from the linear input pot) to then control the output pot in a log-esque way?

[Jerson]

Off the top of my head, you could just use a log lookup table for one of the pots that will return log values instead of linear ones.

[Acetronics2]

Off the top of my head, you could just use a log lookup table for one of the pots that will return log values instead of linear ones.

+1 ...

But care to very poor and variable resolution ...:rolleyes:

Alain

[HankMcSpank]

+1 ...

But care to very poor and variable resolution ...:rolleyes:

Alain

Oop, straight out the blocks, an epic fail on this one!

so to generate a lookup table, I need my digipot's 256 linear step converted to log.

So I use the Excel log function to log 256 number & then charted it within Excel, the problem is, I get a plot like the No4 trace in the first chart/diagram about halfway down this page (ie a reverse log characteristic), whereas what I want is a trace like no2 on that same diagram


http://www.geofex.com/article_folders/potsecrets/potscret.htm

How can I generate my LUT values for a 256 pot & have it look like trace 2?!!! (now I'm sure this is embarrasingly simple ....but I now realise that I shouldn't have tried so hard to pull Mary Harbottle during maths lessons)

[languer]

Try the antilog (i.e. 10^x)

[HankMcSpank]

Try the antilog (i.e. 10^x)

thanks but that wasn't it.

ok, I think I've cracked it, well actually Maxim did for me http://pdfserv.maxim-ic.com/en/an/AN3996.pdf (there's an exe embedded in their Apllication note which yields the attenuation per step - I was able to dick about in Excel until I got my values to match theirs, so for a 256 step pot, you have this formula

Attenuation = 20*log(256 -actual tap_position) / (255)

which yields the cuve in excel I want....albeit in DBs, but I rebased it to actual steps., giving me something like this...



Tap Log Attenuation
1 0 0.0000
2 0 -0.0341
3 0 -0.0684
4 0 -0.1028
5 1 -0.1373
6 1 -0.1720
7 1 -0.2068
8 1 -0.2418
9 1 -0.2769
10 1 -0.3121
11 2 -0.3475
12 2 -0.3830
13 2 -0.4187
14 2 -0.4545
15 2 -0.4905
16 2 -0.5266
17 3 -0.5628
18 3 -0.5993
19 3 -0.6358
20 3 -0.6726
21 3 -0.7094
22 3 -0.7465
23 4 -0.7837
24 4 -0.8210
25 4 -0.8586
26 4 -0.8962
27 4 -0.9341
28 5 -0.9721
29 5 -1.0103
30 5 -1.0486
31 5 -1.0872
32 5 -1.1258
33 5 -1.1647
34 6 -1.2037
35 6 -1.2430
36 6 -1.2823
37 6 -1.3219
38 6 -1.3617
39 7 -1.4016
40 7 -1.4417
41 7 -1.4820
42 7 -1.5225
43 7 -1.5632
44 7 -1.6041
45 8 -1.6452
46 8 -1.6864
47 8 -1.7279
48 8 -1.7695
49 8 -1.8114
50 9 -1.8535
51 9 -1.8957
52 9 -1.9382
53 9 -1.9809
54 9 -2.0238
55 10 -2.0669
56 10 -2.1102
57 10 -2.1537
58 10 -2.1975
59 10 -2.2415
60 11 -2.2857
61 11 -2.3301
62 11 -2.3748
63 11 -2.4197
64 11 -2.4648
65 12 -2.5101
66 12 -2.5557
67 12 -2.6016
68 12 -2.6476
69 13 -2.6940
70 13 -2.7405
71 13 -2.7874
72 13 -2.8344
73 13 -2.8818
74 14 -2.9294
75 14 -2.9772
76 14 -3.0254
77 14 -3.0737
78 15 -3.1224
79 15 -3.1713
80 15 -3.2206
81 15 -3.2700
82 15 -3.3198
83 16 -3.3699
84 16 -3.4202
85 16 -3.4709
86 16 -3.5218
87 17 -3.5731
88 17 -3.6246
89 17 -3.6765
90 17 -3.7286
91 18 -3.7811
92 18 -3.8339
93 18 -3.8871
94 18 -3.9405
95 19 -3.9943
96 19 -4.0484
97 19 -4.1029
98 19 -4.1577
99 20 -4.2128
100 20 -4.2683
101 20 -4.3242
102 20 -4.3804
103 21 -4.4370
104 21 -4.4939
105 21 -4.5513
106 21 -4.6090
107 22 -4.6671
108 22 -4.7256
109 22 -4.7845
110 23 -4.8437
111 23 -4.9034
112 23 -4.9636
113 23 -5.0241
114 24 -5.0850
115 24 -5.1464
116 24 -5.2082
117 25 -5.2705
118 25 -5.3332
119 25 -5.3964
120 25 -5.4600
121 26 -5.5241
122 26 -5.5887
123 26 -5.6538
124 27 -5.7193
125 27 -5.7854
126 27 -5.8519
127 28 -5.9190
128 28 -5.9866
129 28 -6.0547
130 28 -6.1234
131 29 -6.1926
132 29 -6.2624
133 29 -6.3327
134 30 -6.4036
135 30 -6.4751
136 30 -6.5472
137 31 -6.6199
138 31 -6.6932
139 31 -6.7671
140 32 -6.8416
141 32 -6.9168
142 33 -6.9927
143 33 -7.0692
144 33 -7.1464
145 34 -7.2243
146 34 -7.3029
147 34 -7.3823
148 35 -7.4623
149 35 -7.5431
150 35 -7.6247
151 36 -7.7070
152 36 -7.7901
153 37 -7.8741
154 37 -7.9588
155 37 -8.0444
156 38 -8.1308
157 38 -8.2181
158 39 -8.3063
159 39 -8.3954
160 39 -8.4854
161 40 -8.5763
162 40 -8.6682
163 41 -8.7611
164 41 -8.8550
165 42 -8.9500
166 42 -9.0460
167 43 -9.1430
168 43 -9.2412
169 43 -9.3404
170 44 -9.4408
171 44 -9.5424
172 45 -9.6452
173 45 -9.7492
174 46 -9.8545
175 46 -9.9611
176 47 -10.0690
177 47 -10.1783
178 48 -10.2889
179 48 -10.4010
180 49 -10.5145
181 49 -10.6296
182 50 -10.7462
183 51 -10.8643
184 51 -10.9842
185 52 -11.1056
186 52 -11.2288
187 53 -11.3538
188 53 -11.4806
189 54 -11.6093
190 55 -11.7399
191 55 -11.8725
192 56 -12.0072
193 57 -12.1440
194 57 -12.2830
195 58 -12.4242
196 58 -12.5678
197 59 -12.7138
198 60 -12.8622
199 61 -13.0133
200 61 -13.1670
201 62 -13.3235
202 63 -13.4829
203 64 -13.6453
204 64 -13.8107
205 65 -13.9794
206 66 -14.1514
207 67 -14.3269
208 68 -14.5060
209 68 -14.6888
210 69 -14.8756
211 70 -15.0666
212 71 -15.2618
213 72 -15.4614
214 73 -15.6658
215 74 -15.8751
216 75 -16.0896
217 76 -16.3095
218 77 -16.5351
219 78 -16.7668
220 79 -17.0048
221 80 -17.2494
222 81 -17.5012
223 83 -17.7605
224 84 -18.0278
225 85 -18.3036
226 87 -18.5884
227 88 -18.8828
228 89 -19.1876
229 91 -19.5035
230 92 -19.8313
231 94 -20.1720
232 96 -20.5266
233 97 -20.8962
234 99 -21.2823
235 101 -21.6864
236 103 -22.1102
237 105 -22.5557
238 107 -23.0254
239 109 -23.5218
240 112 -24.0484
241 115 -24.6090
242 117 -25.2082
243 120 -25.8519
244 124 -26.5472
245 127 -27.3029
246 131 -28.1308
247 135 -29.0460
248 140 -30.0690
249 145 -31.2288
250 152 -32.5678
251 159 -34.1514
252 168 -36.0896
253 180 -38.5884
254 196 -42.1102
255 224 -48.1308
256 256 Infinity

[Acetronics2]

Hi,

Just confirms #3 ...

try to add a linear, or best 2nd order, regression between each step ...

simple, you said simple ??? ;)

Alain