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Is there anything wrong with driving a comparator differentially

M

MM

Jan 1, 1970
0
Hi all,

I have a circuit where a LT1719 comparator is driven by a secondary of an RF
transformer. The central tap of the transformer is biased to +2.5V. The
comparator is powered from a single +5V supply. The purpose of the circuit
is to square up the input sine wave (at about 50 MHz). Is there anything
wrong with this approach?

The circuit used to work fine but with the recent batch of the parts I
observe the output slew rate being much below the comparator spec especially
when even slightly heated. The shape of the output pulse is also distorted,
there is sort of a step on the rising edge....


Thanks,
/Mikhail
 
T

Tim Wescott

Jan 1, 1970
0
MM said:
Hi all,

I have a circuit where a LT1719 comparator is driven by a secondary of an RF
transformer. The central tap of the transformer is biased to +2.5V. The
comparator is powered from a single +5V supply. The purpose of the circuit
is to square up the input sine wave (at about 50 MHz). Is there anything
wrong with this approach?

The circuit used to work fine but with the recent batch of the parts I
observe the output slew rate being much below the comparator spec especially
when even slightly heated. The shape of the output pulse is also distorted,
there is sort of a step on the rising edge....


Thanks,
/Mikhail
Is the input signal big enough to overcome any input offset voltage and
input overdrive requirements?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/
 
M

MM

Jan 1, 1970
0
Tim Wescott said:
Is the input signal big enough to overcome any input offset voltage and
input overdrive requirements?

The input signal is about 200 mVpp in each leg, i.e. 400 mVpp
differentially, but it is a little noisy...

/Mikhail
 
MM said:
Hi all,

I have a circuit where a LT1719 comparator is driven by a secondary of an RF
transformer. The central tap of the transformer is biased to +2.5V. The
comparator is powered from a single +5V supply. The purpose of the circuit
is to square up the input sine wave (at about 50 MHz). Is there anything
wrong with this approach?

The circuit used to work fine but with the recent batch of the parts I
observe the output slew rate being much below the comparator spec especially
when even slightly heated. The shape of the output pulse is also distorted,
there is sort of a step on the rising edge....

The LT1719 isn't quite fast enough to handle 50MHz reliably - page 15
of the data sheet sets an upper limit of 62,5MHz on the output toggling
frequency. and recommends the L1394 for application requiring fast
toggling - but that part typically falls over at 100MHz, which doesn't
leaving much margin to accomodate worst case parts.

The original Am685 comparator fell over at about 100MHz in the same
sort of way, but the various 9685 follow-on parts got to 200MHz and
above.

I'd be inclined to use a PECL line receiver or two folowed by a fast
PECL-to-TTL converter - the Synergy SY10100ELT23 part is guaranteed to
make 160MHz, and I believe that it was designed by our own Jim
Thompson, whose circuits seem to be okay, even if his politics suggest
incipient Alzheimer's.
 
J

John Larkin

Jan 1, 1970
0
Hi all,

I have a circuit where a LT1719 comparator is driven by a secondary of an RF
transformer. The central tap of the transformer is biased to +2.5V. The
comparator is powered from a single +5V supply. The purpose of the circuit
is to square up the input sine wave (at about 50 MHz). Is there anything
wrong with this approach?

The circuit used to work fine but with the recent batch of the parts I
observe the output slew rate being much below the comparator spec especially
when even slightly heated. The shape of the output pulse is also distorted,
there is sort of a step on the rising edge....


Thanks,
/Mikhail

Use an LVDS-to-TTL line receiver. Dirt cheap and blindingly fast.

John
 
M

MM

Jan 1, 1970
0
John Larkin said:
Use an LVDS-to-TTL line receiver. Dirt cheap and blindingly fast.

John
I'd be inclined to use a PECL line receiver or two folowed by a fast
PECL-to-TTL converter - the Synergy SY10100ELT23 part is guaranteed to
make 160MHz, and I believe that it was designed by our own Jim
Thompson, whose circuits seem to be okay, even if his politics suggest
incipient Alzheimer's.

Guys,

These are all great advices, but it's a production board I am talking about
and I need to fix it without respinning. In all of my new designs I actually
do use PECL and/or LVDS receivers...

Thanks,
/Mikhail
 
F

Fred Bloggs

Jan 1, 1970
0
,
I have a circuit where a LT1719 comparator is driven by a secondary of an RF
transformer. The central tap of the transformer is biased to +2.5V. The
comparator is powered from a single +5V supply. The purpose of the circuit
is to square up the input sine wave (at about 50 MHz). Is there anything
wrong with this approach?

A few things- the LT1719 does not like high impedance drive on the
inputs, which go negative resistance at switch over, such as RF
transformer leakage inductance. You would be better off opening the CT
and placing 2x resistors each of value Rterm/2 in series directly across
the full secondary and running the 2.5V bias to the resistor center tap.
The circuit used to work fine but with the recent batch of the parts I
observe the output slew rate being much below the comparator spec especially
when even slightly heated. The shape of the output pulse is also distorted,
there is sort of a step on the rising edge....

That type of output usually indicates a transient common disruption on
the comparator inputs. The LT1719 likes this on the low end- so you
might consider lowering the 2.5V to more like 1V. There is also a
partial latchup issue at turn on where the LT1719 does not like coming
up with active signals in excess of 200mV being applied to the inputs.
Since the comparator is fully characterized at 20mV overdrive, you might
consider an anti-parallel Schottky clamp fed by nominally small damping
resistors like 10 ohms or so from the input, and bypass the common mode
bias point with a 0.001uF.
The LVDS receiver might be fast but it stinks compared to the LT1719
insofar as sensitivity.
 
MM said:
Guys,

These are all great advices, but it's a production board I am talking about
and I need to fix it without respinning. In all of my new designs I actually
do use PECL and/or LVDS receivers...

If it is a production board, you really shouldn't be using the LT1719 -
to repeat myself

"The LT1719 isn't quite fast enough to handle 50MHz reliably - page 15
of the data sheet sets an upper limit of 62,5MHz on the output toggling

frequency. and recommends the L1394 for application requiring fast
toggling - but that part typically falls over at 100MHz, which doesn't
leave much margin to accomodate worst case parts. "

The 62.5MHz figure in the data sheet is clearly a typical limit, rather
than a worst case guarantee.

Fred Bloggs has come up with an independet problem with the input
stage, which strikes me as an equally valid reason for replacing the
LT1719 with something that can be guaranteed to work.

Production hate it when you change a printed circuit board, but they
hate it even worse when they have to junk a bunch of fully loaded and
tested printed circuit board because one batch of one of the components
came in at the low end of the manufacturer's guaranteed performance
range. When they have to junk boards, they come rampaging through
engineering, looking for scapegoats.
 
M

MM

Jan 1, 1970
0
Thanks a lot Fred. That's the kind of input I was looking for!

/Mikhail
 
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