LTC6820 datasheet(24/92 Pages) AD

24 / 92 page

LTC6811-1/LTC6811-2

Rev. C

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OPERATION

ADC Range and Resolution

The C inputs and GPIO inputs have the same range and

resolution. The ADC inside the LTC6811 has an approxi-

mate range from –0.82V to +5.73V. Negative readings are

rounded to 0V. The format of the data is a 16-bit unsigned

integer where the LSB represents 100µV. Therefore, a

reading of 0x80E8 (33,000 decimal) indicates a measure-

ment of 3.3V.

Delta-SigmaADCshavequantizationnoisewhichdepends

on the input voltage, especially at low oversampling ra-

tios (OSR), such as in FAST mode. In some of the ADC

modes, the quantization noise increases as the input

voltage approaches the upper and lower limits of the

ADC range. For example, the total measurement noise

versus input voltage in normal and filtered modes is

shown in Figure 2.

ADC INPUT VOLTAGE (V)

0.1

0.9

0.7

0.5

0.3

0.8

0.6

0.4

0.2

2.5

4.5

1.5

3.5

68111 F02

0.5

NORMAL MODE

FILTERED MODE

Figure 2. Measurement Noise vs Input Voltage

The specified range of the ADC is 0V to 5V. In Table 4,

the precision range of the ADC is arbitrarily defined as

0.5V to 4.5V. This is the range where the quantization

noise is relatively constant even in the lower OSR modes

(see Figure 2). Table 4 summarizes the total noise in this

range for all eight ADC operating modes. Also shown is

the noise free resolution. For example, 14-bit noise free

resolution in normal mode implies that the top 14 bits will

be noise free with a DC input, but that the 15th and 16th

least significant bits (LSB) will flicker.

Table 4. ADC Range and Resolution

MODE

FULL RANGE1

SPECIFIED

RANGE

PRECISION

RANGE2

LSB

FORMAT

MAX NOISE

NOISE FREE

RESOLUTION3

27kHz (fast)

–0.8192V to

5.7344V

0V to 5V

0.5V to 4.5V

100µV

Unsigned 16 Bits

±4mVP-P

10 Bits

14kHz

±1mVP-P

12 Bits

7kHz (normal)

±250µVP-P

14 Bits

3kHz

±150µVP-P

14 Bits

2kHz

±100µVP-P

15 Bits

1kHz

±100µVP-P

15 Bits

422Hz

±100µVP-P

15 Bits

26Hz (filtered)

±50µVP-P

16 Bits

1. Negative readings are rounded to 0V.

2. PRECISION RANGE is the range over which the noise is less than MAX NOISE.

3. NOISE FREE RESOLUTION is a measure of the noise level within the PRECISION RANGE.

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