LTC2444/LTC2445/
LTC2448/LTC2449
27
2444589fc
For more information www.linear.com/LTC2444
Effective Noise Bandwidth
The LTC2444/LTC2445/LTC2448/LTC2449 has extremely
good input noise rejection from the first notch frequency
all the way out to the modulator sample rate (typically
1.8MHz). Effective noise bandwidth is a measure of how
the ADC will reject wideband input noise up to the modu
-
lator sample rate. The example on the following page
shows how the noise rejection of the LTC2444/LTC2445/
LTC2448/LTC2449 reduces the effective noise of an ampli
-
fier driving its input.
Example: If an amplifier (e.g. LT1219) driving the input of
an LTC2444/LTC2445/LTC2448/LTC2449 has wideband
noise of 33nV/√Hz, band-limited to 1.8MHz, the total noise
entering the ADC input is:
33nV/√Hz • √1.8MHz = 44.3µV.
When the ADC digitizes the input, its digital filter filters
out the wideband noise from the input signal. The noise
reduction depends on the oversample ratio which defines
the effective bandwidth of the digital filter.
At an oversample of 256, the noise bandwidth of the ADC
is 787Hz which reduces the total amplifier noise to:
33nV/√Hz • √787Hz = 0.93µV.
The total noise is the RMS sum of this noise with the 2µV
noise of the ADC at OSR=256.
√(0.93µV)
2
+ (2µV)
2
= 2.2µV.
Increasing the oversample ratio to 32768 reduces the
noise bandwidth of the ADC to 6.2Hz which reduces the
total amplifier noise to:
33nV/√Hz • √6.2Hz = 82nV.
The total noise is the RMS sum of this noise with the
200nV noise of the ADC at OSR = 32768.
√(82nV)
2
+ (200nV)
2
= 216nV.
In this way, the digital filter with its variable oversampling
ratio can greatly reduce the effects of external noise sources.
Automatic Offset Calibration of External
Buffers/Amplifiers
The LTC2445/LTC2449 enable an external amplifier to
be inserted between the multiplexer output and the ADC
input. This enables one external buffer/amplifier circuit to
be
shared between all 17 analog inputs (16 single-ended
or 8 differential). The LTC2445/LTC2449 perform an
internal offset calibration every conversion cycle in order
to remove the offset and drift of the ADC. This calibration
is performed through a combination of front end switch
-
ing and digital processing. Since the external amplifier is
placed between the multiplexer and the ADC, it is inside
the correction loop. This results in automatic offset cor
-
rection and offset drift removal of the external amplifier.
The LT1368 is an excellent amplifier for this function. It
has rail-to-rail inputs and outputs, and it operates on a
single 5V supply. Its open-loop gain is 1M and its input
bias current is 10nA. It also requires at least a 0.1µF load
capacitor for compensation. It is this feature that sets it
apart from other amplifiers—the load capacitor attenuates
sampling glitches from the LTC2445/LTC2449 ADCIN
terminals, allowing it to achieve full performance of the
ADC with high impedance at the multiplexer inputs.
Another benefit of the LT1368 is that it can be powered
from supplies equal to or greater than that of the ADC.
This can allow the inputs to span the entire absolute
maximum of GND – 0.3V to V
CC
+ 0.3V. Using a positive
supply of 7.5V to 10V and a negative supply of –2.5 to –5V
gives the amplifier plenty of headroom over the LTC2445/
LTC2449 input range.
applicaTions inForMaTion
