LMV862MMEVAL National Semiconductor, LMV862MMEVAL Datasheet - Page 17

• Manufacturer Part Number: LMV862MMEVAL
• Manufacturer: National Semiconductor

Specifications of LMV862MMEVAL

• Lead Free Status / Rohs Status: Supplier Unconfirmed

IR PHOTODIODE APPLICATION
The LMV861 and LMV862 are also very good choices to be
used in photodiode applications, such as IR communication,
monitoring, etc. The large bandwidth of the LMV861 and
LM862 makes it possible to create high speed detection. This,
together with the low noise, makes the LMV861 and LMV862
ideal for medical applications such as fetal monitors and bed
side monitors. Another application where the LMV861 and
LMV862 would fit perfectly is a bill validator, an instrument to
detect counterfeit bank notes. The following example de-
scribes an application that can be used for different types of
photodiode sensors and applications.
IR Photodiode Example
The circuit shown in Figure 10 is a typical configuration for the
readout of a photodiode. The response of a photodiode to
incoming light is a variation in the diode current. In many ap-
plications a voltage is required, i.e. when connecting to an
ADC. Therefore the first step is to convert the diode signal
current into a voltage by an I-V converter. In Figure 10 the left
op amp is configured as an I-V converter, with a gain set by
R1.
Some types of photodiodes can have a large capacitance.
This could potentially lead to oscillation. The addition of re-
sistor R2 isolates the photodiode capacitance from the feed-
back loop, thereby preventing the loop from oscillating.
The capacitor in between the two op amp configurations,
blocks the DC component, thus removing the DC offset of the
first op amp circuit, and the offset created by the ambient light
FIGURE 10. IR Photodiode Application
17
entering the photodiode. The second op amp amplifies the
signal to levels that can be converted to a digital signal by an
ADC. To prevent floating of the input of the second op amp,
resistor R5 is added. By allowing the input bias current of a
few pA to flow through this resistor a stable input is ensured.
In Figure 9 a sensed and amplified signal is shown from an
IR source, in this case an IR remote control.
The data from the ADC can then be used by a DSP or micro-
processor for further processing.
FIGURE 9. IR Photodiode Signal
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