+IN

-IN

V

+

V

-

LMP8640

+

-

L

o

a

d

ADC

G

LMP8640

LMP8640HV

www.ti.com

SNOSB28D – AUGUST 2010 – REVISED NOVEMBER 2011

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.

Figure 26. LMP8640 to ADC Interface

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 10m

Ω.

In the table below are summarized the other working condition.

Value

Working Condition

Min

Max

Supply Voltage

5V

5.5V

Common mode Voltage

48V

70V

Temperature

0°C

70°C

Signal BW

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:

considering the maximum monitored current :

For each gain option the max shunt resistors are the following : 20.5m

Ω, 8.2mΩ, 4.1mΩ respectively.

Copyright © 2010–2011, Texas Instruments Incorporated

Submit Documentation Feedback

13

Product Folder Links: LMP8640 LMP8640HV