LTC6804-1/LTC6804-2
23
680412fc
For more information www.linear.com/LTC6804-1
operaTion
The conversion times for these modes are provided in
Table 5. If the core is in STANDBY state, an additional
t
REFUP
time is required to power up the reference before
beginning the ADC conversions. The reference can remain
powered up between ADC conversions if the REFON bit
in Configuration Register Group is set to 1 so the core is
in REFUP state after a delay t
REFUP
. Then, the subsequent
ADC commands will not have the t
REFUP
delay before
beginning ADC conversions.
ADC Range and Resolution
The C inputs and GPIO inputs have the same range and
resolution. The ADC inside the LTC6804 has an approximate
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 measurement of 3.3V.
Delta-Sigma ADCs have quantization noise which depends
on the input voltage, especially at low over sampling ratios
(OSR), such as in FAST mode. In some of the ADC modes,
the quantization noise increases as the input voltage ap
-
proaches 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.
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 six 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.
ADC Range vs Voltage Reference Value:
Typical Delta-Sigma ADC’s have a range which is exactly
twice the value of the voltage reference, and the ADC
measurement error is directly proportional to the error
in the voltage reference. The LTC6804 ADC is not typi
-
cal. The absolute value of V
REF1
is trimmed up or down
to compensate for gain errors in the ADC. Therefore, the
ADC total measurement error (TME) specifications are
superior to the V
REF1
specifications. For example, the
25°C specification of the total measurement error when
measuring 3.300V in 7kHz (normal) mode is ±1.2mV and
the 25°C specification for V
REF1
is 3.200V ±100mV.
Table 4. ADC Range and Resolution
MODE FULL RANGE
1
SPECIFIED
RANGE
PRECISION
RANGE
2
LSB FORMAT MAX NOISE
NOISE FREE
RESOLUTION
3
27kHz (Fast)
–0.8192V to
5.7344V
0V to 5V 0.5V to 4.5V 100µV Unsigned 16 Bits
±4mV
P-P
10 Bits
14kHz ±1mV
P-P
12 Bits
7kHz (Normal) ±250µV
P-P
14 Bits
3kHz ±150µV
P-P
14 Bits
2kHz ±100µV
P-P
15 Bits
26Hz (Filtered) ±50µV
P-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.
Figure 2. Measurement Noise vs Input Voltage
ADC INPUT VOLTAGE (V)
0 0.5
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0
4.03.53.02.52.01.0 1.5 4.5
NORMAL MODE
FILTERED MODE
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