LT1319 Linear Technology, LT1319 Datasheet - Page 6

• Manufacturer Part Number: LT1319
• Description: Multiple Modulation Standard Infrared Receiver
• Manufacturer: Linear Technology

APPLICATIONS
Layout and Passive Components
The LT1319 requires careful layout techniques to minimize
parasitic signal coupling to the preamp input. A sample
board layout for the circuit on the first page is shown in the
Typical Application section. The lead lengths on the photo-
diode must be as short as possible to Pin 2. Shielding is
recommended over the entire circuit. A ground plane must
be used and connected to Pin 1. The ground plane should
extend under the package and surround Pins 1 to 9 and Pin
16. A single point connection should be made to the
ground plane at Pin 12 (DIG_GND). The leads on Pins 6 and
8 should be short to prevent pickup into the gain stages.
The comparator output leads (Pins 10 and 13) should be
as short as possible to minimize coupling back to the input
via parasitic capacitance.
Capacitance on Pin 10 should be minimized as the com-
parator output is pulled up by an internal 5k resistor. The
associated digital circuitry should be located on the oppo-
site side of the PC board from the LT1319 or separated as
much as possible if on the same side of the board. Filter
components should be located on the analog ground side
of the package. Bypass capacitors should be used on Pins
5, 11, 15 and 16 for best supply rejection.
Preamp
The LT1319 preamp is a low noise, high speed current-to-
voltage converter that has been optimized for an input
capacitance of 30pF (which corresponds to the capaci-
tance of the above-mentioned photodiodes with approxi-
mately 2V of back bias). A range of 0pF to 50pF is
acceptable. The amplifier obtains high bandwidth by pro-
viding a low impedance input so that the input current is not
filtered by the photodiode capacitance.
The dynamic range of the circuit will be limited at the low
end by the input-referred current noise of the preamplifier
and the desired signal-to-noise ratio. At the other extreme
of the dynamic range for very large input signals, the output
of the preamp is clamped by Schottky diodes across the
feedback resistor.
The noise bandwidth is shaped by filtering at the output of
the preamplifier and by the AC coupling loop. The input
capacitance causes noise peaking for high bandwidth
applications. Noise peaking can be explained by consider-
LT1319
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INFORMATION
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ing the voltage noise gain. Referring to the Block Diagram,
at frequencies beyond the corner frequency of the AC
coupling loop, the preamp is in a noise gain of 2.5 due to
the ratio of (R
capacitance approaches the same impedance as R
noise gain increases. For example, at 500kHz the 30pF
input capacitance looks like 10.6k which increases the
noise gain to almost 4. The preamp is compensated to
provide a flat current-to-voltage frequency response with a
–3dB corner at 7MHz. The input current noise peaks up
considerably if full bandwidth is used. To obtain best noise
performance, the output of the preamp should be filtered to
the minimum bandwidth required for the desired modula-
tion scheme. The graph of input-referred noise versus
lowpass filtering on the preamp output shows the noise
penalty for higher bandwidths.
AC Coupling Loops
There are three AC loops in the circuit that reject low
frequency inputs. The first loop is around the preamp and
provides rejection of ambient light sources. The operation
can be explained by looking at the Block Diagram. For low
frequency signals the transconductance amplifier, GM1,
compares the preamp output to the V
differential voltage is transformed into a current that is fed
into the high impedance node at Pin 3 and transformed
back to a voltage. There is a voltage gain of approximately
60dB to this point which is then buffered to drive a 10k
resistor that is connected back to the input of the preamp.
This high gain loop attenuates the effect of low frequency
signals by the amount of the loop gain times the ratio of R
to R
cies the attenuation decreases due to the external capacitor
on Pin 3. At frequencies beyond where the loop gain equals
10/15, signals are no longer attenuated. This high fre-
quency cutoff is at:
where 1/(4k ) is the transconductance of the loop ampli-
fier. For example, if C
is 200kHz which can aid in rejection of a wide range of
ambient interference.
The other two loops operate similarly around the gain
stages and also provide low frequency rejection. In addi-
f = (15/10)/(2 • 4k • C
FB
(i.e., 1000V/V • 15/10 = 1500). For higher frequen-
FB
+ R
L1
PIN3
)/R
L1
= 300pF, the highpass frequency
. At high frequencies the input
PIN3
)
BIAS
voltage. This
L1
so the
L1