ADS8881C Datasheet, PDF (33/57 Pages) Texas Instruments – ADS8881x 18-Bit, 1-MSPS, Serial Interface, microPower, Miniature, True-Differential Input, SAR Analog-to-Digital Converter

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ADS8881C, ADS8881I
SBAS547C – MAY 2013 – REVISED JULY 2014
Application Information (continued)
10.1.2.1 Input Amplifier Selection
Selection criteria for the input amplifiers is highly dependent on the input signal type as well as the performance
goals of the data acquisition system. Some key amplifier specifications to consider while selecting an appropriate
amplifier to drive the inputs of the ADC are:
• Small-signal bandwidth. Select the small-signal bandwidth of the input amplifiers to be as high as possible
after meeting the power budget of the system. Higher bandwidth reduces the closed-loop output impedance
of the amplifier, thus allowing the amplifier to more easily drive the low cutoff frequency RC filter (refer to the
Antialiasing Filter section) at the inputs of the ADC. Higher bandwidth also minimizes the harmonic distortion
at higher input frequencies. In order to maintain the overall stability of the input driver circuit, the amplifier
bandwidth should be selected as described in Equation 2:
Unity
 Gain
Bandwidth
t
4 u _{¨¨©§ 2S}
u (R_FLT
1
 R_FLT
) u C_FLT
¸¸¹·
(2)
• Noise. Noise contribution of the front-end amplifiers should be as low as possible to prevent any degradation
in SNR performance of the system. As a rule of thumb, to ensure that the noise performance of the data
acquisition system is not limited by the front-end circuit, the total noise contribution from the front-end circuit
should be kept below 20% of the input-referred noise of the ADC. Noise from the input driver circuit is band-
limited by designing a low cutoff frequency RC filter, as explained in Equation 3.
N_G u
2u
¨§
¨¨©
V₁
f
_ AMP_ PP
6.6
¸·²
¸¸¹

e_{n2 _ RMS} u
S
2
u
f_3dB
d
1
u
V_REF
¨§ ^SNRdB ¸·
u10 ^{© 20 ¹}
52
where:
• V_{1 / f_AMP_PP} is the peak-to-peak flicker noise in µV,
• e_{n_RMS} is the amplifier broadband noise density in nV/√Hz,
• f_–3dB is the 3-dB bandwidth of the RC filter, and
• N_G is the noise gain of the front-end circuit, which is equal to 1 in a buffer configuration.
(3)
• Distortion. Both the ADC and the input driver introduce nonlinearity in a data acquisition block. As a rule of
thumb, to ensure that the distortion performance of the data acquisition system is not limited by the front-end
circuit, the distortion of the input driver should be at least 10 dB lower than the distortion of the ADC, as
shown in Equation 4.
THD_AMP d THD_ADC  10 dB
(4)
• Settling Time. For dc signals with fast transients that are common in a multiplexed application, the input signal
must settle within an 18-bit accuracy at the device inputs during the acquisition time window. This condition is
critical to maintain the overall linearity performance of the ADC. Typically, the amplifier data sheets specify
the output settling performance only up to 0.1% to 0.001%, which may not be sufficient for the desired 18-bit
accuracy. Therefore, the settling behavior of the input driver should always be verified by TINA™-SPICE
simulations before selecting the amplifier.
10.1.2.2 Antialiasing Filter
Converting analog-to-digital signals requires sampling an input signal at a constant rate. Any higher frequency
content in the input signal beyond half the sampling frequency is digitized and folded back into the low-frequency
spectrum. This process is called aliasing. Therefore, an analog, antialiasing filter must be used to remove the
harmonic content from the input signal before being sampled by the ADC. An antialiasing filter is designed as a
low-pass, RC filter, for which the 3-dB bandwidth is optimized based on specific application requirements. For dc
signals with fast transients (including multiplexed input signals), a high-bandwidth filter is designed to allow
accurately settling the signal at the inputs of the ADC during the small acquisition time window. For ac signals,
the filter bandwidth should be kept low to band-limit the noise fed into the input of the ADC, thereby increasing
the signal-to-noise ratio (SNR) of the system.
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