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ADS7846N Datasheet(PDF) 11 Page - Texas Instruments |
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ADS7846N Datasheet(HTML) 11 Page - Texas Instruments |
11 / 30 page ADS7846 11 SBAS125H www.ti.com required for X-position, Y-position, and Z-position measure- ments. Option 3 is to operate in the 15 Clock-per-Conversion mode which overlaps the analog-to-digital conversions and maintains the touch screen drivers on until commanded to stop by the processor (see Figure 12). TEMPERATURE MEASUREMENT In some applications, such as battery recharging, a measure- ment of ambient temperature is required. The temperature measurement technique used in the ADS7846 relies on the characteristics of a semiconductor junction operating at a fixed current level. The forward diode voltage (VBE) has a well-defined characteristic versus temperature. The ambient temperature can be predicted in applications by knowing the 25 °C value of the V BE voltage and then monitoring the delta of that voltage as the temperature changes. The ADS7846 offers two modes of operation. The first mode requires calibration at a known temperature, but only requires a single reading to predict the ambient temperature. The PENIRQ diode is used (turned on) during this measurement cycle. The voltage across the diode is connected through the MUX for digitizing the forward bias voltage by the ADC with an address of A2 = 0, A1 = 0, and A0 = 0 (see Table I and Figure 6 for details). This voltage is typically 600mV at +25 °C with a 20 µA current through the diode. The absolute value of this diode voltage can vary a few millivolts. However, the TC of this voltage is very consistent at –2.1mV/ °C. During the final test of the end product, the diode voltage would be stored at a known room temperature, in memory, for calibration pur- poses by the user. The result is an equivalent temperature measurement resolution of 0.3 °C/LSB (in 12-bit mode). FIGURE 7. Battery Measurement Functional Block Diagram. represented by kT/q • ln (N), where N is the current ratio = 91, k = Boltzmann’s constant (1.38054 • 10–23 electron volts/degrees Kelvin), q = the electron charge (1.602189 • 10–19 C), and T = the temperature in degrees Kelvin. This method can provide improved absolute temperature mea- surement over the first mode at the cost of less resolution (1.6 °C/LSB). The equation for solving for °K is: °K = q • ∆V/(k • ln (N)) (1) where, ∆V = V (I 91) – V (I1) (in mV) ∴ °K = 2.573°K/mV • ∆V °C = 2.573 • ∆V(mV) – 273°K NOTE: The bias current for each diode temperature mea- surement is only on for 3 clock cycles (during the acquisition mode). Therefore, it does not add any noticeable increase in power, especially if the temperature measurement only oc- curs occasionally. BATTERY MEASUREMENT An added feature of the ADS7846 is the ability to monitor the battery voltage on the other side of the voltage regulator (DC/DC converter), as shown in Figure 7. The battery voltage can vary from 0.5V to 6V, while maintaining the voltage to the ADS7846 at 2.7V, 3.3V, etc. The input voltage (VBAT) is divided down by 4 so that a 6.0V battery voltage is represented as 1.5V to the ADC. This simplifies the multiplexer and control logic. In order to minimize the power consumption, the divider is only on during the sampling period when A2 = 0, A1 = 1, and A0 = 0 (see Table I for the relationship between the control bits and configuration of the ADS7846). +V CC V BAT 7.5k Ω 2.5k Ω DC/DC Converter Battery 0.5V to 6.0V 0.125V to 1.5V 2.7V + FIGURE 6. Functional Block Diagram of Temperature Mea- surement Mode. ADC MUX PENIRQ +V CC External Pull-Up X+ Temperature Select TEMP0 TEMP1 The second mode does not require a test temperature calibration, but uses a two-measurement method to eliminate the need for absolute temperature calibration and for achiev- ing 2 °C accuracy. This mode requires a second conversion with an address of A2 = 1, A1 = 1, and A0 = 1, with a 91 times larger current. The voltage difference between the first and second conversion using 91 times the bias current is |
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