Datasheet
CELL
+
-
+
-
ADC
LOAD
+
-
R3
R1
100:
5 k:
V
DD
V
DD
LMV861
LMV861
V
OUT
+
-
LMV861
R2
100:
R4
5 k:
+
-
LMV861
V
DD
V
DD
R5
5 k:
R6
80 k:
P1
20 k:
A1
A2
A3
A4
R6
80 k:
V
REF
V
SENSE
LMV861, LMV862
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SNOSAZ5C –FEBRUARY 2008–REVISED MARCH 2013
Figure 51. Load Cell Application
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
describes an application that can be used for different types of photodiode sensors and applications.
IR Photodiode Example
The circuit shown in Figure 53 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 applications 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 53 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 resistor R2 isolates the photodiode capacitance from the feedback 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 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 52 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 microprocessor for further processing.
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