electrical level conversion

All,

I have previously posted on the differential input circuit that we (Xilinx) use.

I will repeat what I have said before: the differential input circuit is a full CMOS differential comparator. It will operate (function) from rail to rail on its inputs. Its performance has only been characterized for LVDS, and low voltage LVPECL common mode voltages and swings.

Now for the new part: there are no configuration bits to select anything. The comparator is the comparator, and it is the same circuit regardless of standard selected. If it is differential, it is the same circuit.

Hope this helps,

Austin

I have a perl script I posted some time ago that calculates differential impedance (there's a bug I have never fixed in the propagation velocity, though).

I've used this a number of times and empirically it agrees with the board manufacturer's data when they calculate the requirements for me.

Once I get home, I'll post it again.

Cheers

PeteS

# top-level options

# Globals

$Pi = 3.1415926535;

# main print "\n\n"; print "Impedance calculator\n\n"; print "Source material:\n Johnson & Graham, "; print "\"High-Speed Digital Design.\" 1993. Appendix C.\n"; print "Differential impedance calculations based on empirical data\n"; { my ($choice); do { print "\nEnter S for stripline, M for microstrip, X to exit : "; $entry = ; chomp $entry; $choice = uc $entry unless $entry eq ""; print "\n";

if ($choice eq "S") { STRIPLINE(); }

if ($choice eq "M") { MICROSTRIP(); } } until ($choice eq "X") }

# Subroutines BEGIN {

sub E_skny($$$) { local ($h,$w,$Er) = @_; return ($Er + 1) / 2 + ($Er - 1) / 2 * ((1 + 12 * $h / $w) ** -0.5 + 0.04*(1-$w/$h)**2); }

sub E_wide($$$) { local ($h,$w,$Er) = @_; return ($Er + 1) / 2 + ($Er - 1) / 2 * (1 + 12 * $h / $w) ** -0.5; }

sub E_temp($$$) { local ($h, $w, $Er) = @_; return ( $w > $h ? E_wide($h, $w, $Er) : E_skny($h, $w, $Er)); }

sub EEFF($$$$) { local ($h, $w, $t, $Er) = @_; return E_temp($h, $w, $Er) - (($Er - 1) * $t / $h) / 4.6 * ($w / $h) **

0.5; }

sub WE_skny($$$) { local ($h, $w, $t) = @_; return $w + (1.25 * $t)/$Pi * (1 + log(4 * $Pi * $w / $t)); }

sub WE_wide($$$) { local ($h, $w, $t) = @_; return $w + (1.25 * $t)/$Pi * (1 + log(2 * $h / $t)); }

sub WE($$$) { local ($h, $w, $t) = @_; return ($w > $h / (2 * $Pi) ? WE_wide($h, $w, $t) : WE_skny($h, $w, $t)); }

sub ZMS_skny($$$) { local ($h, $w, $t) = @_; return 60 * log((8 * $h / WE($h, $w, $t)) + (WE($h, $w, $t) / (4 * $h))); }

sub ZMS_wide($$$) { local ($h, $w, $t) = @_; return 120 * $Pi / (WE($h, $w, $t) / $h + 1.393 + 0.667 * log(WE($h, $w, $t) / $h + 1.444)); }

sub ZMSTRIP($$$$) { local ($h, $w, $t, $Er) = @_; return ($w > $h ? ZMS_wide($h, $w, $t) : ZMS_skny($h, $w, $t)) / EEFF($h, $w, $t, $Er) ** 0.5; }

sub PMSTRIP($$$$) { local ($h, $w, $t, $Er) = @_; return 84.72e-12 * EEFF($h, $w, $t, $Er) ** 0.5; }

sub LMSTRIP($$$$) { local ($h, $w, $t, $x) = @_; return PMSTRIP($h, $w, $t, 1) * ZMSTRIP($h, $w, $t, 1) * $x; }

sub CMSTRIP($$$$$) { local ($h, $w, $t, $Er, $x) = @_; return PMSTRIP($h, $w, $t, $Er) / ZMSTRIP($h, $w, $t, $Er) * $x; }

{ # this block makes the "my" below permanent my ($h, $w, $t, $s); sub MICROSTRIP { print "\nMicrostrip configuration\n"; $h *= 1000; $w *= 1000; $t *= 1000; $s *= 1000; print "Enter height to reference plane (mil) : "; $input = ; $h = $input unless $input eq "\n"; print "Enter width of track (mil) : "; $input = ; $w = $input unless $input eq "\n"; print "Enter separation of diff traces : "; $input = ; $s = $input unless $input eq "\n"; print "Enter thickness of trace (mil) (1oz = 1.37) : "; $input = ; $t = $input unless $input eq "\n"; print "Enter dielectric constant : "; $input = ; $Er = $input unless $input eq "\n";

$h /= 1000; $w /= 1000; $t /= 1000; $s /= 1000; chomp $Er; $hi_accuracy = 0; if (($t / $h > 0) && ($t / $h < 0.2)) { if (($w / $h > 0.1) && ($w / $h < 20)) { if (($Er > 0) && ($Er < 16)) { $hi_accuracy = 1; } } }

print "\n"; $Zo = ZMSTRIP($h, $w, $t, $Er); printf "Calculated impedance (ohm): %6.2f ", $Zo; print ($hi_accuracy ? "(accuracy > 98%)" : "(accuracy undefined)"); print "\n"; printf "Differential impedance (ohm): %6.2f (+/-10%)\n", ZMDIFF ($Zo, $s, $h); printf "Calculated prop delay (in/ns): %6.2f\n", 1e-9 / PMSTRIP($h, $w, $t, $Er); printf "Calculated inductance (nH/in): %6.2f\n", 1e9 * LMSTRIP($h, $w, $t, 1); printf "Calculated capacitance (pF/in): %6.2f\n", 1e12 * CMSTRIP($h, $w, $t, $Er, 1); } }

sub ZSTR_K1 ($$) { local ($w, $t) = @_; local ($tmp1, $tmp2, $tmp3) = 0; $tmp1 = 1 + log(4 * $Pi * $w / $t); $tmp2 = 1 + $t * $tmp1 / ($Pi * $w); $tmp3 = $tmp2 + 0.255 * ($t / $w) ** 2; return $w * $tmp3 / 2; }

sub ZSTR_skny ($$$$) { local ($b, $w, $t, $Er) = @_; return 60 / ($Er ** 0.5) * log (4 * $b / ($Pi * ZSTR_K1($w, $t))); }

sub ZSTR_K2 ($$) { local ($b, $t) = @_; local ($tmp1) = 0; $tmp1 = 1 - $t / $b; return 2 / $tmp1 * log(1 / $tmp1 + 1) - (1 / $tmp1 - 1) * log(1 / $tmp1 ** 2 - 1); }

sub ZSTR_wide ($$$$) { local ($b, $w, $t, $Er) = @_; return 94.15 / $Er ** 0.5 / ( ($w / $b) / (1 - $t / $b) + ZSTR_K2 ($b, $t) / $Pi); }

sub ZSTRIP ($$$$) { local ($b, $w, $t, $Er) = @_; return ($w > 0.35 * $b ? ZSTR_wide($b, $w, $t, $Er) : ZSTR_skny ($b, $w, $t, $Er)); }

sub ZOFFSET ($$$$$) { local ($h1, $h2, $w, $t, $Er) = @_; return (2 * ZSTRIP(2 * $h1 + $t, $w, $t, $Er) * ZSTRIP(2 * $h2 + $t, $w, $t, $Er)) / (ZSTRIP(2 * $h1 + $t, $w, $t, $Er) + ZSTRIP(2 * $h2 + $t, $w, $t, $Er)); }

sub PSTRIP($) { local $Er = @_; return 84.72e-12 * $Er ** 0.5; }

sub LSTRIP($$$$) { local ($b, $w, $t, $x) = @_; return PSTRIP(1) * ZSTRIP($b, $w, $t, 1) * $x; }

sub LOSTRIP($$$$$) { local ($h1, $h2, $w, $t, $x) = @_; return PSTRIP(1) * ZOFFSET($h1, $h2, $w, $t, 1) * $x; }

sub CSTRIP($$$$$) { local ($b, $w, $t, $Er, $x) = @_; return PSTRIP($Er) / ZSTRIP($b, $w, $t, $Er); }

sub COSTRIP($$$$$$) { local ($h1, $h2, $w, $t, $Er, $x) = @_; return PSTRIP($Er) / ZOFFSET($h1, $h2, $w, $t, $Er); }

{ # this line makes the "my" below permanent my ($h1, $h2, $w, $t, $s); sub STRIPLINE { print "\nStripline configuration\n"; $h1 *= 1000; $h2 *= 1000; $w *= 1000; $t *= 1000; $s *= 1000; print "Enter distance to reference plane above (mil) : "; $input = ; $h1 = $input unless $input eq "\n"; print "Enter distance to reference plane below (mil) : "; $input = ; $h2 = $input unless $input eq "\n"; print "Enter width of track (mil) : "; $input = ; $w = $input unless $input eq "\n"; print "Enter separation of diff traces : "; $input = ; $s = $input unless $input eq "\n"; print "Enter thickness of trace (mil) (1oz = 1.37) : "; $input = ; $t = $input unless $input eq "\n"; print "Enter dielectric constant : "; $input = ; $Er = $input unless $input eq "\n";

$h1 /= 1000; $h2 /= 1000; $w /= 1000; $t /= 1000; $s /= 1000; chomp $Er;

$b = $h1 + $h2 + $t; $hi_accuracy = 0;

if ($h1 == $h2) { print "Symmetric stripline configuration\n\n"; if (($t / $b < 0.25) && ($t / $w < 0.11)) { $hi_accuracy = 1 } $symmetric = 1; } else { print "Asymmetric stripline configuration\n\n"; $symmetric = 0; }

$Zo = ($symmetric ? ZSTRIP($b,$w,$t,$Er) : ZOFFSET($h1, $h2, $w, $t, $Er)); printf "Calculated impedance (ohm): %6.2f ", $Zo; print ($hi_accuracy ? "(accuracy > 98.7%)" : "(accuracy undefined)"); print "\n"; printf "Differential impedance (ohm): %6.2f (+/-10%)\n", ZSDIFF($Zo, $s, $h1, $h2); printf "Calculated prop delay (in/ns): %6.2f\n", 1e-9 / PSTRIP($Er); printf "Calculated inductance (nH/in): %6.2f\n", 1e9 * ($symmetric ? LSTRIP($b, $w, $t, 1) : LOSTRIP($h1, $h2, $w, $t, 1)); printf "Calculated capacitance (pF/in): %6.2f\n", 1e12 * ($symmetric ? CSTRIP($b, $w, $t, $Er, 1) : COSTRIP($h1, $h2, $w, $t, $Er, 1)); } } sub ZMDIFF ($$$) { my ($Zo, $s, $h) = @_; return 2*$Zo*(1-0.48*exp(-.96*$s/$h)); } sub ZSDIFF ($$$$) { my ($Zo, $s, $h1, $h2) = @_; return 2*$Zo*(1-0.374*exp(-2.9*$s/($h1+$h2))); } }

It is, incidentally, a *very good* comparator!

John