6

LT1319

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

noise gain increases. For example, at 500kHz the 30pF

input capacitance looks like 10.6k

Ω 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,

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

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:

f = (15/10)/(2

where 1/(4k

Ω) is the transconductance of the loop ampli-

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-

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-