Datasheet

Instrumentation
Amplifier
Mux
Low-Pass
Filters
Ch x
Ch3
Ch2
Ch1
S1
S2
S3
Sx
Ideal
Actual
ChangeofMux
OutputVoltage
Ch1
Ch2
Currentinto
ProtectionClamp
LongSettlingof
VoltageonCh2
100 s/divm
5V/div
1V/div
t
2
t
1
MuxSwitching
InputstoPGA
withBufferOFF
InputstoPGA
withBufferON
BufferActivated
PGA280
www.ti.com ................................................................................................................................................ SBOS487A –JUNE 2009–REVISED SEPTEMBER 2009
Note: Current from the protection clamp into the signal source and filter produces a long settling delay.
Figure 46. Typical Block Diagram for Multiplexed Data Acquisition
Together with the switching command of the multiplexer or internal switching, the current buffers (BUF) can be
activated to prevent such clamp currents. The buffers do not have clamps as long as the signal remains within
the supply boundaries. Figure 47 shows an example of the input signal settling for both conditions: without and
with the buffer activated.
Without the buffer, there is an obvious long settling, depending on signal and filter impedance. With the buffer
activated, only the amplifier has to settle and no distorting current is reflected into the signal source and filter; no
glitch is visible in this plot. The plot shows the resulting settling of the input signal for a positive and a negative
signal step as indicated in Figure 46; also shown are the SPI signal and the BUFA signal.
Figure 47. Example for Amplifier Settling Without (t
1
) and With (t
2
) Buffer (BUF) Activated
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