AMC7836IPAPR datasheet(73/88 Pages) TI1

73 / 88 page

VCC

RSP

RSN

VOUT

GND

ADC Input

DAC Voltage

PC GPIO

Drain

Gate

Source

R(SENSE)

PAVDD

LMP8480

AMC7836

www.ti.com

SLAS986C – NOVEMBER 2014 – REVISED APRIL 2016

Product Folder Links: AMC7836

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Application Information (continued)

8.1.2 Current Sensing Applications

In applications that require current sensing of the power amplifier, an external high-side current sense amplifier

can be added and configured to the unipolar ADC inputs. Figure 116 shows this design.

The LMP8480 device is a precision current sense device that amplifies the small differential voltage developed

across a current-sense resistor in the presence of high input common-mode voltages. The LMP8480 device

accepts input signals with a common-mode voltage range from 4 V to 76 V with a bandwidth of 270 kHz. The

LMP8480 device offers different fixed gain settings. The optimal gain setting is dependent on the accuracy

requirement of the application. To maintain precision over temperature, the output of the LMP8480 device should

be directly connected to the AMC7836 unipolar ADC inputs. If the output range of the LMP8480 device is scaled

by a voltage divider, as shown in Figure 116, an output amplifier may be required to drive the ADC unipolar input

to ensure a low impedance source. If the series resistance, in this case R4, is low enough then the buffer may

not be required because the LMP8480 device is capable of driving the input of the AMC7836 unipolar ADC

channel.

NOTE

The external resistors will cause some small error because of temperature drift and the

input bias current of the operation amplifier.

Figure 116 also shows a simple method to ensure proper power sequencing of the power amplifier by adding a

series PMOS transistor to the PA drain terminal. The activation of the PMOS transistor connects the PAVDD

voltage supply to the drain pin of the power amplifier. The PMOS transistor is driven with a voltage divider that

swings from the PAVDD voltage to PAVDD × (R2 / (R1 + R2)). The NMOS shown in Figure 116 is connected to a

microcontroller output that controls the state of the PMOS transistor.

Figure 116. Current-Sense Application With PMOS ON and OFF

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