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Any idea about Spice funny?

I've been getting some funnies with LTSpice (freshly downloaded version) which seem to centre round this trivial circuit: just a differential amplifier with a gain of 33 (if it looks a bit odd, it's because it's extracted from the original circuit). I'd rather naively expect the output to be a 3.33V 1kHz sinewave with a 7.5V offset. Instead of which I get an initial spike of -27kV, which ramps at about 1.6V/us. Which doesn't look right to me. It also takes an age to simulate, so much so that I haven't had the patience to get to the running bit yet.

If I try to do an AC analysis I just get iteration limit. Again, the circuit is pretty simple, I'd expect it to do this one, so i must be doing something silly.

I hardly use Spice, but I'm sure it should be easier than this. Anyone have any idea what egregious blunder I'm making? Any kind help for a mostly digital bloke much appreciated.

Here's the source:

Version 4 SHEET 1 880 680 WIRE 608 0 304 0 WIRE 304 80 304 0 WIRE 448 80 384 80 WIRE 96 144 -160 144 WIRE 176 224 0 224 WIRE 384 224 384 160 WIRE 384 224 256 224 WIRE 608 272 608 0 WIRE -160 288 -160 144 WIRE 96 288 96 144 WIRE 384 304 384 224 WIRE 384 304 128 304 WIRE 0 320 0 224 WIRE 64 320 0 320 WIRE 304 336 304 160 WIRE 304 336 128 336 WIRE 304 352 304 336 WIRE 304 448 304 432 WIRE -160 544 -160 368 WIRE 96 544 96 352 WIRE 96 544 -160 544 WIRE 304 544 304 528 WIRE 304 544 96 544 WIRE 448 544 448 80 WIRE 448 544 304 544 WIRE 608 544 608 352 WIRE 608 544 448 544 FLAG 0 224 VIFB SYMBOL Opamps\\\\opamp2 96 256 M0 SYMATTR InstName U1 SYMATTR Value TLC081 SYMBOL res 288 64 R0 SYMATTR InstName R1 SYMATTR Value 10k SYMBOL res 288 336 R0 SYMATTR InstName R2 SYMATTR Value 333k SYMBOL res 272 208 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R3 SYMATTR Value 333k SYMBOL res 368 64 R0 SYMATTR InstName R4 SYMATTR Value 10k SYMBOL voltage -160 272 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL voltage 608 256 R0 WINDOW 3 24 160 Left 0 WINDOW 123 24 132 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value SINE(0 .1 1000) SYMATTR Value2 AC .1 SYMBOL voltage 304 432 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value 7.5 TEXT -144 64 Left 0 !.tran 1 TEXT -152 8 Left 0 !.inc TLC081.mod

Here's the TLC081 (TLC081.mod):

  • TL081 OPERATIONAL AMPLIFIER "MACROMODEL" SUBCIRCUIT
  • CREATED USING PARTS RELEASE 4.01 ON 06/16/89 AT 13:08
  • (REV N/A) SUPPLY VOLTAGE: +/-15V
  • CONNECTIONS: NON-INVERTING INPUT
  • | INVERTING INPUT
  • | | POSITIVE POWER SUPPLY
  • | | | NEGATIVE POWER SUPPLY
  • | | | | OUTPUT
  • | | | | | .SUBCKT TL081 1 2 3 4 5
  • C1 11 12 3.498E-12 C2 6 7 15.00E-12 DC 5 53 DX DE 54 5 DX DLP 90 91 DX DLN 92 90 DX DP 4 3 DX EGND 99 0 POLY(2) (3,0) (4,0) 0 .5 .5 FB 7 99 POLY(5) VB VC VE VLP VLN 0 4.715E6 -5E6 5E6 5E6 -5E6 GA 6 0 11 12 282.8E-6 GCM 0 6 10 99 8.942E-9 ISS 3 10 DC 195.0E-6 HLIM 90 0 VLIM 1K J1 11 2 10 JX J2 12 1 10 JX R2 6 9 100.0E3 RD1 4 11 3.536E3 RD2 4 12 3.536E3 RO1 8 5 150 RO2 7 99 150 RP 3 4 2.143E3 RSS 10 99 1.026E6 VB 9 0 DC 0 VC 3 53 DC 2.200 VE 54 4 DC 2.200 VLIM 7 8 DC 0 VLP 91 0 DC 25 VLN 0 92 DC 25 .MODEL DX D(IS=800.0E-18) .MODEL JX PJF(IS=15.00E-12 BETA=270.1E-6 VTO=-1) .ENDS 
Reply to
Paul Burke
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Reposted because I forgot the important bit, sorry!!!

I've been getting some funnies with LTSpice (freshly downloaded version) which seem to centre round this trivial circuit: just a differential amplifier with a gain of 33 (if it looks a bit odd, it's because it's extracted from the original circuit). I'd rather naively expect the output to be a 3.33V 1kHz sinewave with a 7.5V offset. Instead of which I get an initial spike of -27kV, which ramps at about 1.6V/us. Which doesn't look right to me. It also takes an age to simulate, so much so that I haven't had the patience to get to the running bit yet.

If I try to do an AC analysis I just get iteration limit. Again, the circuit is pretty simple, I'd expect it to do this one, so i must be doing something silly.

I hardly use Spice, but I'm sure it should be easier than this. Anyone have any idea what egregious blunder I'm making? Any kind help for a mostly digital bloke much appreciated.

Here's the source:

  • C:\Program Files\LTC\SwCADIII\Draft5.asc XU1 N005 N004 N003 N006 VIFB TLC081 R1 N001 N005 10k R2 N005 N002 333k R3 N004 VIFB 333k R4 N002 N004 10k V1 N003 N006 15 V2 N001 N002 SINE(0 .1 1000) AC .1 V3 N002 N006 7.5 ;tran 1 startup .inc TLC081.mod .ac oct 5 1 100k .inc op37g.cir .backanno .end

********************************** The LTCSwitcherCad bit:

Version 4 SHEET 1 880 680 WIRE 608 0 304 0 WIRE 304 80 304 0 WIRE 448 80 384 80 WIRE 96 144 -160 144 WIRE 176 224 0 224 WIRE 384 224 384 160 WIRE 384 224 256 224 WIRE 608 272 608 0 WIRE -160 288 -160 144 WIRE 96 288 96 144 WIRE 384 304 384 224 WIRE 384 304 128 304 WIRE 0 320 0 224 WIRE 64 320 0 320 WIRE 304 336 304 160 WIRE 304 336 128 336 WIRE 304 352 304 336 WIRE 304 448 304 432 WIRE -160 544 -160 368 WIRE 96 544 96 352 WIRE 96 544 -160 544 WIRE 304 544 304 528 WIRE 304 544 96 544 WIRE 448 544 448 80 WIRE 448 544 304 544 WIRE 608 544 608 352 WIRE 608 544 448 544 FLAG 0 224 VIFB SYMBOL Opamps\\\\opamp2 96 256 M0 SYMATTR InstName U1 SYMATTR Value TLC081 SYMBOL res 288 64 R0 SYMATTR InstName R1 SYMATTR Value 10k SYMBOL res 288 336 R0 SYMATTR InstName R2 SYMATTR Value 333k SYMBOL res 272 208 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R3 SYMATTR Value 333k SYMBOL res 368 64 R0 SYMATTR InstName R4 SYMATTR Value 10k SYMBOL voltage -160 272 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL voltage 608 256 R0 WINDOW 3 24 160 Left 0 WINDOW 123 24 132 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value SINE(0 .1 1000) SYMATTR Value2 AC .1 SYMBOL voltage 304 432 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value 7.5 TEXT -144 64 Left 0 !.tran 1 TEXT -152 8 Left 0 !.inc TLC081.mod

**************************************** Here's the TLC081 (TLC081.mod):

  • TL081 OPERATIONAL AMPLIFIER "MACROMODEL" SUBCIRCUIT

  • CREATED USING PARTS RELEASE 4.01 ON 06/16/89 AT 13:08
  • (REV N/A) SUPPLY VOLTAGE: +/-15V
  • CONNECTIONS: NON-INVERTING INPUT
  • | INVERTING INPUT
  • | | POSITIVE POWER SUPPLY
  • | | | NEGATIVE POWER SUPPLY
  • | | | | OUTPUT
  • | | | | | .SUBCKT TL081 1 2 3 4 5
  • C1 11 12 3.498E-12 C2 6 7 15.00E-12 DC 5 53 DX DE 54 5 DX DLP 90 91 DX DLN 92 90 DX DP 4 3 DX EGND 99 0 POLY(2) (3,0) (4,0) 0 .5 .5 FB 7 99 POLY(5) VB VC VE VLP VLN 0 4.715E6 -5E6 5E6 5E6 -5E6 GA 6 0 11 12 282.8E-6 GCM 0 6 10 99 8.942E-9 ISS 3 10 DC 195.0E-6 HLIM 90 0 VLIM 1K J1 11 2 10 JX J2 12 1 10 JX R2 6 9 100.0E3 RD1 4 11 3.536E3 RD2 4 12 3.536E3 RO1 8 5 150 RO2 7 99 150 RP 3 4 2.143E3 RSS 10 99 1.026E6 VB 9 0 DC 0 VC 3 53 DC 2.200 VE 54 4 DC 2.200 VLIM 7 8 DC 0 VLP 91 0 DC 25 VLN 0 92 DC 25 .MODEL DX D(IS=800.0E-18) .MODEL JX PJF(IS=15.00E-12 BETA=270.1E-6 VTO=-1) .ENDS 
Reply to
Paul Burke

[snip]

I didn't run it, but there's no GROUND (Node 0) in the schematic, so that's likely your problem.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

"Jim Thompson" schrieb im Newsbeitrag news: snipped-for-privacy@4ax.com...

Hello Paul,

This missing ground is indeed the reason for the convergence problem.

The next mistake is using the TL081 so very close(100mV) to the negative supply rail. This will lead to a lot of distortion i the simulation and in a real circuit too. Just add a negative supply voltage and the distortion will disappear.

Why do you provide a TL081 model while using a TLC081 in the schematic?

Best regards, Helmut

PS: If you want do a THD calculation, use the following setting.

.tran 0 0.01 0 10u .options plotwinsize=0 .four 1k V(VIFB)

The .four result can be viewed with the following menu-comamnd.

View -> SPICE Errorlog

Reply to
Helmut Sennewald

Reply to
Paul Burke

That is the problem, thanks. There was one on the circuit I abstracted in from, and it didn't copy, and I didn't notice.

Paul Burke

Reply to
Paul Burke

I kept changing the opamp trying to get it to work, and the problem with the original circuit was operation too close to the power rails. Next, to find an opamp that will take 15V supply (plus a reasonable margin) AND do rail to rail IO, or change the circuit to work around a midpoint reference.

I must practise this analog stuff more.

Paul Burke

Reply to
Paul Burke

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