LM4F111E5QR Datasheet, PDF (741/1114 Pages) Texas Instruments – Microcontroller

Stellaris^® LM4F111E5QR Microcontroller
The voltage sampled in differential mode is the difference between the odd and even channels:
■ Input Positive Voltage: VIN+ = V_{IN_EVEN} (even channel)
■ Input Negative Voltage: VIN- = V_{IN_ODD} (odd channel)
The input differential voltage is defined as: VIN_D = VIN+ - VIN-, therefore:
■ If VIN_D = 0, then the conversion result = 0x800
■ If VIN_D > 0, then the conversion result > 0x800 (range is 0x800–0xFFF)
■ If VIN_D < 0, then the conversion result < 0x800 (range is 0–0x800)
When using differential sampling, the following definitions are relevant:
■ Input Common Mode Voltage: VIN_CM = (VIN+ + VIN-) / 2
■ Reference Positive Voltage: VREFP
■ Reference Negative Voltage: VREFN
■ Reference Differential Voltage: VREF_D = VREFP - VREFN
■ Reference Common Mode Voltage: VREF_CM = (VREFP + VREFN) / 2
The following conditions provide optimal results in differential mode:
■ Both V_{IN_EVEN} and V_{IN_ODD} must be in the range of (VREFP to VREFN) for a valid conversion
result
■ The maximum possible differential input swing, or the maximum differential range, is: -VREF_Dto
+VREF_D, so the maximum peak-to-peak input differential signal is (+VREF_D - -VREF_D) = 2 *
VREF_D= 2 * (VREFP - VREFN)
■ In order to take advantage of the maximum possible differential input swing, VIN_CM should be
very close to VREF_CM, see Table 21-21 on page 1065.
If VIN_CM is not equal to VREF_CM, the differential input signal may clip at either maximum or minimum
voltage, because either single ended input can never be larger than VREFP or smaller than VREFN,
and it is not possible to achieve full swing. Thus any difference in common mode between the input
voltage and the reference voltage limits the differential dynamic range of the ADC.
Because the maximum peak-to-peak differential signal voltage is 2 * (VREFP - VREFN), the ADC
codes are interpreted as:
mV per ADC code = (2 *(VREFP - VREFN)) / 4096
Figure 12-10 shows how the differential voltage, ∆V, is represented in ADC codes.
April 25, 2012
741
Texas Instruments-Advance Information