Datasheet

LMP91000
www.ti.com SNAS506H –JANUARY 2011–REVISED MARCH 2013
Control amplifier
The control amplifier (A1 op amp in Figure 25) has two tasks: a) providing initial charge to the sensor, b)
providing a bias voltage to the sensor. A1 has the capability to drive up to 10mA into the sensor in order to to
provide a fast initial conditioning. A1 is able to sink and source current according to the connected gas sensor
(reducing or oxidizing gas sensor). It can be powered down to reduce system power consumption. However
powering down A1 is not recommended, as it may take a long time for the sensor to recover from this situation.
Variable Bias
The Variable Bias block circuitry (Figure 25) provides the amount of bias voltage required by a biased gas sensor
between its reference and working electrodes. The bias voltage can be programmed to be 1% to 24% (14 steps
in total) of the supply, or of the external reference voltage. The 14 steps can be programmed through the I
2
C
interface. The polarity of the bias can be also programmed.
Internal zero
The internal Zero is the voltage at the non-inverting pin of the TIA. The internal zero can be programmed to be
either 67%, 50% or 20%, of the supply, or the external reference voltage. This provides both sufficient headroom
for the counter electrode of the sensor to swing, in case of sudden changes in the gas concentration, and best
use of the ADC’s full scale input range.
The Internal zero is provided through an internal voltage divider (Vref divider box in Figure 25). The divider is
programmed through the I
2
C interface.
Temperature sensor
The embedded temperature sensor can be switched off during gas concentration measurement to save power.
The temperature measurement is triggered through the I
2
C interface. The temperature output is available at the
VOUT pin until the configuration bit is reset. The output signal of the temperature sensor is a voltage, referred to
the ground of the LMP91000 (AGND).
Table 1. Temperature Sensor Transfer
Temperature Output Voltage Temperature Output Voltage
(°C) (mV) (°C) (mV)
-40 1875 23 1375
-39 1867 24 1367
-38 1860 25 1359
-37 1852 26 1351
-36 1844 27 1342
-35 1836 28 1334
-34 1828 29 1326
-33 1821 30 1318
-32 1813 31 1310
-31 1805 32 1302
-30 1797 33 1293
-29 1789 34 1285
-28 1782 35 1277
-27 1774 36 1269
-26 1766 37 1261
-25 1758 38 1253
-24 1750 39 1244
-23 1742 40 1236
-22 1734 41 1228
-21 1727 42 1220
-20 1719 43 1212
-19 1711 44 1203
-18 1703 45 1195
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