that1250 THAT Corporation, that1250 Datasheet - Page 4

• Manufacturer Part Number: that1250
• Description: Low Cost, Balanced Line Receiver Ics
• Manufacturer: THAT Corporation

Document 600068 Rev 02
demanding applications in which the source imped-
ance balance may be less than perfect, the 1200-
series ICs offer exceptional CMRR performance via a
patented method of increasing common-mode input
impedance.
Input Considerations
against input overload via an unusual arrangement of
diodes connecting the + and - Input pins to the
power supply pins. The circuit of Figure 3 shows the
arrangement used for the R
applies to the other side. The zener diodes prevent
the protection network from conducting until an
input pin is raised at least 50 V above V
V
devices without constraining the allowable signal
swing at the input pins. The reference (and sense)
pins are protected via more conventional reverse-
biased diodes which will conduct if these pins are
raised above V
stages, their input pins are of necessity connected to
the outside world. This is likely to expose the parts
to ESD when cables are connected and disconnected.
Our testing indicates that the 1250-series devices will
typically withstand application of up to 1,000 volts
under the human body ESD model.
prevent RF from reaching the devices, THAT recom-
mends the circuit of Figure 4. C
be located close to the point where the input signal
comes into the chassis, preferably directly on the
input
minimizes the unbalancing impact of differences in
the values of C
from each input to chassis ground to depend primar-
ily on the value of C
mately ten times less sensitive to mismatches
between C
approach in which the junction of C
grounded directly
ing through the addition of R-C networks at the
inputs (series resistor followed by a capacitor to
ground at each input). Generally, THAT recommends
keeping any such series resistances under 50W, so
as not to upset the intrinsic balance between the
1250’s internal R
Because the internal resistor absolute values are not
well controlled, the external resistors can interact
with the internal ones in unexpected ways. As an
alternative to a resistor as additional build-out
impedance, THAT recommends the use of a ferrite
bead or balun instead.
1250-series parts, the coupling capacitors should be
sized to present negligible impedance at any frequen-
cies of interest for common mode rejection. Regard-
less of the type of coupling capacitor chosen,
variations in the values of the two capacitors,
working against the 1250-series input impedance,
can unbalance common mode input signals, convert-
ing them to balanced signals which will not be
rejected by the CMRR of the devices. For this reason,
EE
. Thus, the protection networks protect the
The 1250-series devices are internally protected
Because the 1250-series devices are input
To reduce risk of damage from ESD, and to
Designers frequently seek to improve RF bypass-
If it is necessary to ac-couple the inputs of the
connector.
4
and C
CC
4
or below V
6
and C
.
1
/R
5
3
The
. The circuit shown is approxi-
2
than the more conventional
5
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
and R
THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
by forcing the capacitance
unusual
EE
3
.
/ R
3
4
/R
3
side; a similar one
through C
4
resistor ratios.
circuit
Copyright © 2008, THAT Corporation
4
CC
and C
or below
5
should
design
5
is
Page 4 of 8
THAT recommends dc-coupling the inputs of the
1250-series devices.
Input Voltage Limitations
-6 dB gain, the 1253 and 1256 devices are capable of
accepting input signals above the power supply rails.
This is because the internal opamp’s inputs connect
to the outside world only through the on-chip resis-
tors R
Figure 2. Consider the following analysis.
Differential Input Signals
signal handling will be output clipping. The outputs
of all the devices typically clip at within 2V of the
supply rails. Therefore, maximum differential input
signal levels are directly related to the gain and
supply rails.
Common-mode Input Signals
little output signal. The limitation on common-mode
handling is the point at which the inputs are
overloaded. So, we must consider the inputs of the
opamp.
mode input current splits to flow through both R
and through R
follow V
(R
Furthermore, the same constraints apply to Va as in
the differential analysis.
maximum common-mode input signal for the 1250 is
(2V
the 1253, these figures are (2.4V
(2.4V
(3V
rails, the 1250 will accept up to ~26 V in either
direction. As an ac signal, this is 52 V peak-peak,
18.4 V rms, or +27.5 dBu. With the same supply
rails, the 1253 will accept up to ~31 V in either
direction. As an ac signal, this is 62 V peak-peak,
21.9 V rms, or +29 dBu. With the same supply rails,
the 1256 will accept up to ~39 V in either direction.
As an ac signal, this is 78 V peak-peak, 27.6 V rms,
or +31 dBu.
common-mode signals combine. The maximum
signal that can be accommodated will depend on the
3
CC
CC
+ R
When configured, respectively, for -3 dB and
For differential signals (v
For common-mode input signals, there is very
For common-mode signals (V
The voltage at a can be calculated as:
Again, solving for V
For the 1250, (R
Following the same reasoning as above, the
Therefore, for common-mode signals and ±15 V
Of course, in the real world, differential and
v
v
EE
- 6) V, and (3V
- 4) V, and the minimum is (2V
a
IN(CM)
1
4
a
= v
+ 4.8) V.
through R
) / R
, we will consider only the voltage at node a.
IN(CM)
= v
4
= 2.4. For the 1256, (R
3
a
/R
4
R
4
Low-cost Balanced Line Receiver ICs
.
R
3
R
For the 1256, these figures are
EE
+R
3
at nodes a and b as shown in
R
4
+R
3
4
Because v
+ 6) V.
4
+ R
4
IN(CM)
.
.
4
) / R
,
IN(DIFF)
4
b
= 2. For the 1253,
IN(CM)
), the limitation to
is constrained to
CC
THAT 1250 Series
), the common-
EE
3
- 4.8) V, and
+ R
+ 4) V. For
4
) / R
4
1
=3.
/R
2