REF5045 datasheet(34/62 Pages) TI2

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THD

ADC

AMP

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SNR

REF

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AM P

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FLT

Bandwidth

Gain

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ADS8881

SBAS547D – MAY 2013 – REVISED AUGUST 2015

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Application Information (continued)

11.1.2.1 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 (see 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, select the

amplifier bandwidth as described in Equation 2:

(2)

•

Noise. Noise contribution of the front-end amplifiers must 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

must 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.

where:

•

V1 / f_AMP_PP is the peak-to-peak flicker noise in µV,

•

en_RMS is the amplifier broadband noise density in nV/√Hz,

•

f–3dB is the 3-dB bandwidth of the RC filter, and

•

NG 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 must be at least 10 dB lower than the distortion of the ADC, as shown

in Equation 4.

(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, always verify the settling behavior of the input driver by TINA™-SPICE simulations

before selecting the amplifier.

11.1.2.2 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,

keep the filter bandwidth 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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