SNOSB28F – AUGUST 2010 – REVISED APRIL 2013
DRIVING ADC
The input stage of an Analog to Digital converter can be modeled with a resistor and a capacitance versus
ground. So if the voltage source doesn't have a low impedance an error in the amplitude's measurement will
occur. In this case a buffer is needed to drive the ADC. The LMP8640 has an internal output buffer able to drive
a capacitance load up to 30 pF or the input stage of an ADC. If required an external low pass RC filter can be
added at the output of the LMP8640 to reduce the noise and the bandwidth of the current sense.
I
S
+IN
R
IN
R
S
-IN
R
IN
LMP8640
L
o
a
d
+
-
+
-
V
+
R
F
G
V
OUT
R
G
= 2*R
IN
V
-
Figure 26. LMP8640 to ADC Interface
+
-
V
A
ADC
C
F
DESIGN EXAMPLE
For example in a current monitor application is required to measure the current sunk by a load (peak current
10A) with a resolution of 10mA and 0.5% of accuracy. The 10bit analog to digital converter accepts a max input
voltage of 4.1V. Moreover in order to not burn much power on the shunt resistor it needs to be less than 10mΩ.
In the table below are summarized the other working condition.
Working Condition
Supply Voltage
Common mode Voltage
Temperature
Signal BW
Value
Min
5V
48V
0°C
Max
5.5V
70V
70°C
50kHz
First step – LMP8640 / LMP8640HV selection
The required common mode voltage of the application implies that the right choice is the LMP8640HV (High
common mode voltage up tp 76V).
Second step – Gain option selection
We can choose between three gain option (20V/V, 50V/V, 100V/V). considering the max input voltage of the
ADC (4.1V) , the max Sense voltage across the shunt resistor is evaluated according the following formula:
V
SENSE
= (MAX Vin ADC) / Gain;
hence the max V
SENSE
will be 205mV, 82mV, 41mV respectively. The shunt resistor are then evaluated
considering the maximum monitored current :
R
S
= (max V
SENSE
) / I_MAX
For each gain option the max shunt resistors are the following : 20.5mΩ, 8.2mΩ, 4.1mΩ respectively.
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13
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