Datasheet

VIN2
VIN4*
VIN3*
VREFN1
VIN6/VREFP2
ADC
VREFP1
+
-
+
-
+
-
VIN0
VIN5*
VIN1
VIN7/VREFN2
VINP
VINN
BUFF
FGA
* VIN3, VIN4, VIN5 are only available for LMP90080 and LMP90079
LMP90077, LMP90078, LMP90079, LMP90080
SNAS521F –JULY 2011–REVISED MARCH 2013 www.ti.com
Figure 31. Simplified VIN Circuitry
Selectable Gains (FGA & PGA)
LMP900xx provides two types of gain amplifiers: a fixed gain amplifier (FGA) and a programmable gain amplifier
(PGA). FGA has a fixed gain of 16x or it can be bypassed, while the PGA has programmable gain settings of 1x,
2x, 4x, or 8x.
Total gain is defined as FGA x PGA. Thus, LMP900xx provides gain settings of 1x, 2x, 4x, 8x, 16x, 32x, 64x, or
128x with true continuous background calibration.
The gain is channel specific, which means that one channel can have one gain, while another channel can have
the same or a different gain.
The gain can be selected by programming the CHx_CONFIG: GAIN_SEL bits.
Buffer (BUFF)
There is an internal unity gain buffer that can be included or excluded from the signal path. Including the buffer
provides a high input impedance but increases the power consumption.
When gain ≥ 16, the buffer is automatically included in the signal path. When gain < 16, including or excluding
the buffer from the signal path can be done by programming the CHX_CONFIG: BUF_EN bit.
Internal/External CLK Selection
LMP900xx allows two clock options: internal CLK or external CLK (crystal (XTAL) or clock source).
There is an “External Clock Detection” mode, which detects the external XTAL if it is connected to XOUT and
XIN. When operating in this mode, the LMP900xx shuts off the internal clock to reduce power consumption.
Below is a flow chart to help set the appropriate clock registers.
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