LTspice, it\'s been awhile...

R

Ricky

Guest
I\'m simulating a rather simple circuit, but am not getting the appropriate results on AC analysis. I must be forgetting something about how to do an AC analysis.

I run a transient simulation and get what I expect. When the switch is open (5V on the control pin), the signal is attenuated four fold. When the switch is closed (0V on the control pin), the signal is attenuated 16 fold.

But run an AC analysis and the signal \"Filter\" is shown as -24 dB instead of th expected -12 dB (with the switch open). Close the switch and the signal \"Filter\" is still -24 dB, but in this case, that\'s expected.

If I change the connection from the net -13dBCntl to a ground, instead of the switch, it works as I expect.

I don\'t get why the transient simulation shows the expected results, yet the AC analysis does not. Obviously the switch model is munging up the AC analysis, but why???

Here\'s the circuit file.

Version 4
SHEET 1 880 684
WIRE -176 112 -208 112
WIRE -32 112 -176 112
WIRE 160 112 48 112
WIRE 256 112 160 112
WIRE 368 112 256 112
WIRE 416 112 368 112
WIRE 480 112 416 112
WIRE 544 112 480 112
WIRE 656 112 624 112
WIRE 672 112 656 112
WIRE 160 176 160 112
WIRE 256 176 256 112
WIRE 368 176 368 112
WIRE 480 176 480 112
WIRE -208 192 -208 112
WIRE 16 224 -16 224
WIRE 112 224 96 224
WIRE 112 256 112 224
WIRE 368 288 368 256
WIRE 480 288 480 240
WIRE 672 288 672 112
WIRE -16 304 -16 224
WIRE -208 368 -208 272
WIRE -112 368 -208 368
WIRE 160 368 160 240
WIRE 160 368 80 368
WIRE 208 368 160 368
WIRE 256 368 256 256
WIRE 256 368 208 368
WIRE -208 384 -208 368
WIRE -112 432 -208 432
WIRE -208 480 -208 432
WIRE -208 592 -208 560
WIRE -64 608 -64 544
WIRE 32 608 32 544
WIRE 32 608 16 608
WIRE 64 608 32 608
WIRE 64 640 64 608
FLAG -208 384 0
FLAG 480 288 0
FLAG 672 288 0
FLAG -208 592 0
FLAG 64 640 0
FLAG 112 256 0
FLAG 368 288 0
FLAG 656 112 Out
FLAG 416 112 Filter
FLAG -176 112 In
FLAG 208 368 -13dBCntl
SYMBOL res 64 96 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R1
SYMATTR Value 5k
SYMBOL res 240 160 R0
SYMATTR InstName R2
SYMATTR Value 417
SYMBOL res 640 96 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R4
SYMATTR Value 51k
SYMBOL cap 464 176 R0
SYMATTR InstName C1
SYMATTR Value 1.5nF
SYMBOL cap 144 176 R0
SYMATTR InstName C2
SYMATTR Value 4.7nF
SYMBOL Switches\\\\ADG1411 -16 416 R0
SYMATTR InstName U1
SYMBOL voltage -208 176 R0
WINDOW 123 24 124 Left 2
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value SINE(1 1 1000)
SYMATTR Value2 AC 1
SYMBOL voltage -208 464 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 5
SYMBOL voltage 0 224 R270
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value 12
SYMBOL voltage 32 608 R90
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V4
SYMATTR Value 12
SYMBOL res 352 160 R0
SYMATTR InstName R3
SYMATTR Value 1650
TEXT 302 540 Left 2 ;.tran 10m
TEXT 304 464 Left 2 !.ac dec 20 1 100k


--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209
 
On Thursday, February 23, 2023 at 12:43:35 AM UTC-8, Ricky wrote:
I\'m simulating a rather simple circuit, but am not getting the appropriate results on AC analysis. I must be forgetting something about how to do an AC analysis.

I run a transient simulation and get what I expect. When the switch is open (5V on the control pin), the signal is attenuated four fold. When the switch is closed (0V on the control pin), the signal is attenuated 16 fold.

But run an AC analysis and the signal \"Filter\" is shown as -24 dB instead of th expected -12 dB (with the switch open). Close the switch and the signal \"Filter\" is still -24 dB, but in this case, that\'s expected.

If I change the connection from the net -13dBCntl to a ground, instead of the switch, it works as I expect.

I don\'t get why the transient simulation shows the expected results, yet the AC analysis does not. Obviously the switch model is munging up the AC analysis, but why???

Here\'s the circuit file.

Version 4
SHEET 1 880 684
WIRE -176 112 -208 112
WIRE -32 112 -176 112
WIRE 160 112 48 112
WIRE 256 112 160 112
WIRE 368 112 256 112
WIRE 416 112 368 112
WIRE 480 112 416 112
WIRE 544 112 480 112
WIRE 656 112 624 112
WIRE 672 112 656 112
WIRE 160 176 160 112
WIRE 256 176 256 112
WIRE 368 176 368 112
WIRE 480 176 480 112
WIRE -208 192 -208 112
WIRE 16 224 -16 224
WIRE 112 224 96 224
WIRE 112 256 112 224
WIRE 368 288 368 256
WIRE 480 288 480 240
WIRE 672 288 672 112
WIRE -16 304 -16 224
WIRE -208 368 -208 272
WIRE -112 368 -208 368
WIRE 160 368 160 240
WIRE 160 368 80 368
WIRE 208 368 160 368
WIRE 256 368 256 256
WIRE 256 368 208 368
WIRE -208 384 -208 368
WIRE -112 432 -208 432
WIRE -208 480 -208 432
WIRE -208 592 -208 560
WIRE -64 608 -64 544
WIRE 32 608 32 544
WIRE 32 608 16 608
WIRE 64 608 32 608
WIRE 64 640 64 608
FLAG -208 384 0
FLAG 480 288 0
FLAG 672 288 0
FLAG -208 592 0
FLAG 64 640 0
FLAG 112 256 0
FLAG 368 288 0
FLAG 656 112 Out
FLAG 416 112 Filter
FLAG -176 112 In
FLAG 208 368 -13dBCntl
SYMBOL res 64 96 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R1
SYMATTR Value 5k
SYMBOL res 240 160 R0
SYMATTR InstName R2
SYMATTR Value 417
SYMBOL res 640 96 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R4
SYMATTR Value 51k
SYMBOL cap 464 176 R0
SYMATTR InstName C1
SYMATTR Value 1.5nF
SYMBOL cap 144 176 R0
SYMATTR InstName C2
SYMATTR Value 4.7nF
SYMBOL Switches\\\\ADG1411 -16 416 R0
SYMATTR InstName U1
SYMBOL voltage -208 176 R0
WINDOW 123 24 124 Left 2
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value SINE(1 1 1000)
SYMATTR Value2 AC 1
SYMBOL voltage -208 464 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 5
SYMBOL voltage 0 224 R270
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value 12
SYMBOL voltage 32 608 R90
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V4
SYMATTR Value 12
SYMBOL res 352 160 R0
SYMATTR InstName R3
SYMATTR Value 1650
TEXT 302 540 Left 2 ;.tran 10m
TEXT 304 464 Left 2 !.ac dec 20 1 100k
Click to expand...

The switch is a time domain device. Perhaps it is because of the initial conditions. ac analysis starts with the dc operating point (an initialization). It (ac) doesn\'t do anything beyond time 0. It is unclear what is inside the ADG1411 model. It probably does not initialize as you are expecting for t=0. If I replace it with an ltspice switch, the 12 dB response occurs.

Version 4
SHEET 1 880 904
WIRE -176 112 -208 112
WIRE -32 112 -176 112
WIRE 160 112 48 112
WIRE 256 112 160 112
WIRE 368 112 256 112
WIRE 416 112 368 112
WIRE 480 112 416 112
WIRE 544 112 480 112
WIRE 656 112 624 112
WIRE 672 112 656 112
WIRE 160 176 160 112
WIRE 256 176 256 112
WIRE 368 176 368 112
WIRE 480 176 480 112
WIRE -208 192 -208 112
WIRE 16 224 -16 224
WIRE 112 224 96 224
WIRE 112 256 112 224
WIRE 368 288 368 256
WIRE 480 288 480 240
WIRE 672 288 672 112
WIRE -16 496 -16 224
WIRE -208 560 -208 272
WIRE -112 560 -208 560
WIRE 160 560 160 240
WIRE 192 560 160 560
WIRE 224 560 192 560
WIRE 256 560 256 256
WIRE 256 560 224 560
WIRE -208 576 -208 560
WIRE -112 624 -208 624
WIRE -208 672 -208 624
WIRE 224 672 224 560
WIRE 336 688 272 688
WIRE 336 736 336 688
WIRE 224 768 224 752
WIRE 272 768 272 736
WIRE 272 768 224 768
WIRE -208 784 -208 752
WIRE 224 784 224 768
WIRE -64 800 -64 736
WIRE 32 800 32 736
WIRE 32 800 16 800
WIRE 64 800 32 800
WIRE 64 832 64 800
WIRE 336 832 336 816
FLAG -208 576 0
FLAG 480 288 0
FLAG 672 288 0
FLAG -208 784 0
FLAG 64 832 0
FLAG 112 256 0
FLAG 368 288 0
FLAG 656 112 Out
FLAG 416 112 Filter
FLAG -176 112 In
FLAG 192 560 -13dBCntl
FLAG 224 784 0
FLAG 336 832 0
SYMBOL res 64 96 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R1
SYMATTR Value 5k
SYMBOL res 240 160 R0
SYMATTR InstName R2
SYMATTR Value 417
SYMBOL res 640 96 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R4
SYMATTR Value 51k
SYMBOL cap 464 176 R0
SYMATTR InstName C1
SYMATTR Value 1.5nF
SYMBOL cap 144 176 R0
SYMATTR InstName C2
SYMATTR Value 4.7nF
SYMBOL Switches\\\\ADG1411 -16 608 R0
SYMATTR InstName U1
SYMBOL voltage -208 176 R0
WINDOW 123 24 124 Left 2
WINDOW 39 0 0 Left 0
SYMATTR Value2 AC 1
SYMATTR InstName V1
SYMATTR Value SINE(1 1 1000)
SYMBOL voltage -208 656 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 5
SYMBOL voltage 0 224 R270
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value 12
SYMBOL voltage 32 800 R90
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V4
SYMATTR Value 12
SYMBOL res 352 160 R0
SYMATTR InstName R3
SYMATTR Value 1650
SYMBOL sw 224 768 R180
SYMATTR InstName S1
SYMATTR Value SW_S1
SYMBOL voltage 336 720 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V5
SYMATTR Value 0
TEXT 304 544 Left 2 ;.tran 10m
TEXT 304 464 Left 2 !.ac dec 20 1 100k
TEXT 160 888 Left 2 !.model SW_S1 SW(Ron=1u Roff=10Meg Vt=0.5 Vh=0)
 
On Thursday, February 23, 2023 at 3:43:35 AM UTC-5, Ricky wrote:
I\'m simulating a rather simple circuit, but am not getting the appropriate results on AC analysis. I must be forgetting something about how to do an AC analysis.

I run a transient simulation and get what I expect. When the switch is open (5V on the control pin), the signal is attenuated four fold. When the switch is closed (0V on the control pin), the signal is attenuated 16 fold.

But run an AC analysis and the signal \"Filter\" is shown as -24 dB instead of th expected -12 dB (with the switch open). Close the switch and the signal \"Filter\" is still -24 dB, but in this case, that\'s expected.

If I change the connection from the net -13dBCntl to a ground, instead of the switch, it works as I expect.

I don\'t get why the transient simulation shows the expected results, yet the AC analysis does not. Obviously the switch model is munging up the AC analysis, but why???

Here\'s the circuit file.

Version 4
SHEET 1 880 684
WIRE -176 112 -208 112
WIRE -32 112 -176 112
WIRE 160 112 48 112
WIRE 256 112 160 112
WIRE 368 112 256 112
WIRE 416 112 368 112
WIRE 480 112 416 112
WIRE 544 112 480 112
WIRE 656 112 624 112
WIRE 672 112 656 112
WIRE 160 176 160 112
WIRE 256 176 256 112
WIRE 368 176 368 112
WIRE 480 176 480 112
WIRE -208 192 -208 112
WIRE 16 224 -16 224
WIRE 112 224 96 224
WIRE 112 256 112 224
WIRE 368 288 368 256
WIRE 480 288 480 240
WIRE 672 288 672 112
WIRE -16 304 -16 224
WIRE -208 368 -208 272
WIRE -112 368 -208 368
WIRE 160 368 160 240
WIRE 160 368 80 368
WIRE 208 368 160 368
WIRE 256 368 256 256
WIRE 256 368 208 368
WIRE -208 384 -208 368
WIRE -112 432 -208 432
WIRE -208 480 -208 432
WIRE -208 592 -208 560
WIRE -64 608 -64 544
WIRE 32 608 32 544
WIRE 32 608 16 608
WIRE 64 608 32 608
WIRE 64 640 64 608
FLAG -208 384 0
FLAG 480 288 0
FLAG 672 288 0
FLAG -208 592 0
FLAG 64 640 0
FLAG 112 256 0
FLAG 368 288 0
FLAG 656 112 Out
FLAG 416 112 Filter
FLAG -176 112 In
FLAG 208 368 -13dBCntl
SYMBOL res 64 96 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R1
SYMATTR Value 5k
SYMBOL res 240 160 R0
SYMATTR InstName R2
SYMATTR Value 417
SYMBOL res 640 96 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R4
SYMATTR Value 51k
SYMBOL cap 464 176 R0
SYMATTR InstName C1
SYMATTR Value 1.5nF
SYMBOL cap 144 176 R0
SYMATTR InstName C2
SYMATTR Value 4.7nF
SYMBOL Switches\\\\ADG1411 -16 416 R0
SYMATTR InstName U1
SYMBOL voltage -208 176 R0
WINDOW 123 24 124 Left 2
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value SINE(1 1 1000)
SYMATTR Value2 AC 1
SYMBOL voltage -208 464 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 5
SYMBOL voltage 0 224 R270
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value 12
SYMBOL voltage 32 608 R90
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V4
SYMATTR Value 12
SYMBOL res 352 160 R0
SYMATTR InstName R3
SYMATTR Value 1650
TEXT 302 540 Left 2 ;.tran 10m
TEXT 304 464 Left 2 !.ac dec 20 1 100k


--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209
Click to expand...

If you replace the ADG1411 with an ADG1412, the AC and DC analysis both work (with the switch operation reversed).
There seems to be something wrong with the ADG1411 model.

--Marc
 
On Thursday, February 23, 2023 at 11:16:08 PM UTC-4, Marc Guardiani wrote:
On Thursday, February 23, 2023 at 3:43:35 AM UTC-5, Ricky wrote:
I\'m simulating a rather simple circuit, but am not getting the appropriate results on AC analysis. I must be forgetting something about how to do an AC analysis.

I run a transient simulation and get what I expect. When the switch is open (5V on the control pin), the signal is attenuated four fold. When the switch is closed (0V on the control pin), the signal is attenuated 16 fold.

But run an AC analysis and the signal \"Filter\" is shown as -24 dB instead of th expected -12 dB (with the switch open). Close the switch and the signal \"Filter\" is still -24 dB, but in this case, that\'s expected.

If I change the connection from the net -13dBCntl to a ground, instead of the switch, it works as I expect.

I don\'t get why the transient simulation shows the expected results, yet the AC analysis does not. Obviously the switch model is munging up the AC analysis, but why???

Here\'s the circuit file.

Version 4
SHEET 1 880 684
WIRE -176 112 -208 112
WIRE -32 112 -176 112
WIRE 160 112 48 112
WIRE 256 112 160 112
WIRE 368 112 256 112
WIRE 416 112 368 112
WIRE 480 112 416 112
WIRE 544 112 480 112
WIRE 656 112 624 112
WIRE 672 112 656 112
WIRE 160 176 160 112
WIRE 256 176 256 112
WIRE 368 176 368 112
WIRE 480 176 480 112
WIRE -208 192 -208 112
WIRE 16 224 -16 224
WIRE 112 224 96 224
WIRE 112 256 112 224
WIRE 368 288 368 256
WIRE 480 288 480 240
WIRE 672 288 672 112
WIRE -16 304 -16 224
WIRE -208 368 -208 272
WIRE -112 368 -208 368
WIRE 160 368 160 240
WIRE 160 368 80 368
WIRE 208 368 160 368
WIRE 256 368 256 256
WIRE 256 368 208 368
WIRE -208 384 -208 368
WIRE -112 432 -208 432
WIRE -208 480 -208 432
WIRE -208 592 -208 560
WIRE -64 608 -64 544
WIRE 32 608 32 544
WIRE 32 608 16 608
WIRE 64 608 32 608
WIRE 64 640 64 608
FLAG -208 384 0
FLAG 480 288 0
FLAG 672 288 0
FLAG -208 592 0
FLAG 64 640 0
FLAG 112 256 0
FLAG 368 288 0
FLAG 656 112 Out
FLAG 416 112 Filter
FLAG -176 112 In
FLAG 208 368 -13dBCntl
SYMBOL res 64 96 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R1
SYMATTR Value 5k
SYMBOL res 240 160 R0
SYMATTR InstName R2
SYMATTR Value 417
SYMBOL res 640 96 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R4
SYMATTR Value 51k
SYMBOL cap 464 176 R0
SYMATTR InstName C1
SYMATTR Value 1.5nF
SYMBOL cap 144 176 R0
SYMATTR InstName C2
SYMATTR Value 4.7nF
SYMBOL Switches\\\\ADG1411 -16 416 R0
SYMATTR InstName U1
SYMBOL voltage -208 176 R0
WINDOW 123 24 124 Left 2
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value SINE(1 1 1000)
SYMATTR Value2 AC 1
SYMBOL voltage -208 464 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 5
SYMBOL voltage 0 224 R270
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value 12
SYMBOL voltage 32 608 R90
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V4
SYMATTR Value 12
SYMBOL res 352 160 R0
SYMATTR InstName R3
SYMATTR Value 1650
TEXT 302 540 Left 2 ;.tran 10m
TEXT 304 464 Left 2 !.ac dec 20 1 100k


--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209
If you replace the ADG1411 with an ADG1412, the AC and DC analysis both work (with the switch operation reversed).
There seems to be something wrong with the ADG1411 model.

--Marc
Click to expand...

Thanks. Seems odd, one having a fundamental flaw and the other not when the only difference is the polarity of the digital controls. They also have a third variation (1413) with two controls of one polarity and two the other. Because these parts, and the similar ones made by Maxim and TI are all in short supply, I plan to use a resistor install option to flag the FPGA which of the three are being used. The CM will hate having so many different BoMs, some 8 or 9 variations from this one part selection alone. The other parts with multiple variations will just have to select the part used, without other changes. I find CMs get confused with this stuff very easily, but I guess I can understand that. They just want to build boards, not learn how they work.

--

Rick C.

+ Get 1,000 miles of free Supercharging
+ Tesla referral code - https://ts.la/richard11209
 
On Thursday, February 23, 2023 at 3:51:35 PM UTC-4, Simon S Aysdie wrote:
On Thursday, February 23, 2023 at 12:43:35 AM UTC-8, Ricky wrote:
I\'m simulating a rather simple circuit, but am not getting the appropriate results on AC analysis. I must be forgetting something about how to do an AC analysis.

I run a transient simulation and get what I expect. When the switch is open (5V on the control pin), the signal is attenuated four fold. When the switch is closed (0V on the control pin), the signal is attenuated 16 fold.

But run an AC analysis and the signal \"Filter\" is shown as -24 dB instead of th expected -12 dB (with the switch open). Close the switch and the signal \"Filter\" is still -24 dB, but in this case, that\'s expected.

If I change the connection from the net -13dBCntl to a ground, instead of the switch, it works as I expect.

I don\'t get why the transient simulation shows the expected results, yet the AC analysis does not. Obviously the switch model is munging up the AC analysis, but why???

Here\'s the circuit file.

Version 4
SHEET 1 880 684
WIRE -176 112 -208 112
WIRE -32 112 -176 112
WIRE 160 112 48 112
WIRE 256 112 160 112
WIRE 368 112 256 112
WIRE 416 112 368 112
WIRE 480 112 416 112
WIRE 544 112 480 112
WIRE 656 112 624 112
WIRE 672 112 656 112
WIRE 160 176 160 112
WIRE 256 176 256 112
WIRE 368 176 368 112
WIRE 480 176 480 112
WIRE -208 192 -208 112
WIRE 16 224 -16 224
WIRE 112 224 96 224
WIRE 112 256 112 224
WIRE 368 288 368 256
WIRE 480 288 480 240
WIRE 672 288 672 112
WIRE -16 304 -16 224
WIRE -208 368 -208 272
WIRE -112 368 -208 368
WIRE 160 368 160 240
WIRE 160 368 80 368
WIRE 208 368 160 368
WIRE 256 368 256 256
WIRE 256 368 208 368
WIRE -208 384 -208 368
WIRE -112 432 -208 432
WIRE -208 480 -208 432
WIRE -208 592 -208 560
WIRE -64 608 -64 544
WIRE 32 608 32 544
WIRE 32 608 16 608
WIRE 64 608 32 608
WIRE 64 640 64 608
FLAG -208 384 0
FLAG 480 288 0
FLAG 672 288 0
FLAG -208 592 0
FLAG 64 640 0
FLAG 112 256 0
FLAG 368 288 0
FLAG 656 112 Out
FLAG 416 112 Filter
FLAG -176 112 In
FLAG 208 368 -13dBCntl
SYMBOL res 64 96 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R1
SYMATTR Value 5k
SYMBOL res 240 160 R0
SYMATTR InstName R2
SYMATTR Value 417
SYMBOL res 640 96 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R4
SYMATTR Value 51k
SYMBOL cap 464 176 R0
SYMATTR InstName C1
SYMATTR Value 1.5nF
SYMBOL cap 144 176 R0
SYMATTR InstName C2
SYMATTR Value 4.7nF
SYMBOL Switches\\\\ADG1411 -16 416 R0
SYMATTR InstName U1
SYMBOL voltage -208 176 R0
WINDOW 123 24 124 Left 2
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value SINE(1 1 1000)
SYMATTR Value2 AC 1
SYMBOL voltage -208 464 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 5
SYMBOL voltage 0 224 R270
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value 12
SYMBOL voltage 32 608 R90
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V4
SYMATTR Value 12
SYMBOL res 352 160 R0
SYMATTR InstName R3
SYMATTR Value 1650
TEXT 302 540 Left 2 ;.tran 10m
TEXT 304 464 Left 2 !.ac dec 20 1 100k
The switch is a time domain device. Perhaps it is because of the initial conditions. ac analysis starts with the dc operating point (an initialization). It (ac) doesn\'t do anything beyond time 0. It is unclear what is inside the ADG1411 model. It probably does not initialize as you are expecting for t=0. If I replace it with an ltspice switch, the 12 dB response occurs..
Click to expand...

Thanks. That makes some sense. But as Marc pointed out, the ADG1412 model works with the AC simulation. I guess I should jump through the hoops and report this. I don\'t recall how difficult ADI makes it to report bugs. Some companies make you jump through all manner of hoops.

--

Rick C.

-- Get 1,000 miles of free Supercharging
-- Tesla referral code - https://ts.la/richard11209
 
On Thursday, February 23, 2023 at 11:16:08 PM UTC-4, Marc Guardiani wrote:
On Thursday, February 23, 2023 at 3:43:35 AM UTC-5, Ricky wrote:
I\'m simulating a rather simple circuit, but am not getting the appropriate results on AC analysis. I must be forgetting something about how to do an AC analysis.

I run a transient simulation and get what I expect. When the switch is open (5V on the control pin), the signal is attenuated four fold. When the switch is closed (0V on the control pin), the signal is attenuated 16 fold.

But run an AC analysis and the signal \"Filter\" is shown as -24 dB instead of th expected -12 dB (with the switch open). Close the switch and the signal \"Filter\" is still -24 dB, but in this case, that\'s expected.

If I change the connection from the net -13dBCntl to a ground, instead of the switch, it works as I expect.

I don\'t get why the transient simulation shows the expected results, yet the AC analysis does not. Obviously the switch model is munging up the AC analysis, but why???

Here\'s the circuit file.

snipped
Click to expand...

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209
If you replace the ADG1411 with an ADG1412, the AC and DC analysis both work (with the switch operation reversed).
There seems to be something wrong with the ADG1411 model.

--Marc
Click to expand...

I had posted about this at LTspice groups.io. The first couple of replies complained that I had manual grounds instead of the switches connected. Then someone who was sharp enough to figure that out said that the model was problematic, but could be fixed by adding a FET to the schematic, any FET, no need to be connected to anything. LOL

He had parameterized the simulation and I have done the initial measurements I needed. It would be nice to have the -3 dB measurements made automatically, along with the gains, but manual is good enough I suppose. I\'m really only trying to verify my manual calculations.

I\'ve always hated parts of LTspice. The UI is amazingly clumsy if you have not used it in a while. The response to the scroll wheel is not linear. Spin it a bit too fast and the zooms in or out far too much! Virtually every control is the opposite of what is intuitive (where intuitive likely means, I\'m used to it because that\'s what 99% of the other software I use does).

But, it does work... mostly, and it allows me to get the job done, even if it\'s like shaving with cold water and a rusty blade.

--

Rick C.

-+ Get 1,000 miles of free Supercharging
-+ Tesla referral code - https://ts.la/richard11209
 
Ok, so now I\'ve added some scripting to automate the -3dB measurements. It works, but I can\'t figure out how to control the lower cutoff from the upper cutoff. The first parametric run doesn\'t include the -3 dB cutoff at the low end, so it reports the low pass cutoff at the high frequency end. The other steps measure the low frequency, high pass cutoff. Is there a way to tell it to scan from high to low, instead of low to high? Or can it be given a frequency to start scanning from? I guess I can dig into the bowels of the documentation, but \"bowels\" is a good term for it. The .meas command is very complex and hard to interpret the documentation. I\'ve been down this road before.

--

Rick C.

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+- Tesla referral code - https://ts.la/richard11209
 
On Saturday, February 25, 2023 at 5:40:33 AM UTC-4, Ricky wrote:
> Ok, so now I\'ve added some scripting to automate the -3dB measurements. It works, but I can\'t figure out how to control the lower cutoff from the upper cutoff. The first parametric run doesn\'t include the -3 dB cutoff at the low end, so it reports the low pass cutoff at the high frequency end. The other steps measure the low frequency, high pass cutoff. Is there a way to tell it to scan from high to low, instead of low to high? Or can it be given a frequency to start scanning from? I guess I can dig into the bowels of the documentation, but \"bowels\" is a good term for it. The .meas command is very complex and hard to interpret the documentation. I\'ve been down this road before.

Opps, forgot the script.

..meas AC Filter_max MAX MAG(V(Filter))
..meas AC Filter_3dB PARAM Filter_max/sqrt(2)
..meas AC fmax_3dB FIND freq WHEN MAG(V(Filter))=Filter_3dB

--

Rick C.

++ Get 1,000 miles of free Supercharging
++ Tesla referral code - https://ts.la/richard11209
 
The simulation is working pretty well at this point. The circuit is a bit groady, with multiple specs that apply in different configurations. So there\'s a specific order which has to be followed to design all the details. It\'s nice to have a simulation to confirm you haven\'t bollocksed it up.

Now I need to run it by the customer to confirm they are happy with the results. I just hope there\'s enough room on the board for the added chips. I was able to remove on op amp by simplifying the amp section. The new requirement is for a lower voltage output, so I can eliminate the more complex positive feedback, synthetic impedance circuit. We\'ll see if the various changes make enough room to get the new stuff on. It\'s a tiny board.

--

Rick C.

--- Get 1,000 miles of free Supercharging
--- Tesla referral code - https://ts.la/richard11209
 
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