ADC14X250 datasheet(44/69 Pages) TI1

44 / 69 page

VIN+

VIN-

VCM

ADC

0.1uF

10uF

10 :

MABA007159

0.01uF

0.1uF

33 :

Z0 = 50 :

ADC14X250

SLASE49 – DECEMBER 2015

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The insertion loss between an ADC driver and the ADC input is important because the driver must overcome the

insertion loss of the connecting network to drive the ADC to full-scale and achieve the best SNR. Minimizing the

loss through the network reduces the output swing and distortion requirements of the driver and usually

translates to a system-level power savings in the driver. This can be accomplished by selecting transformers or

filter designs with low insertion loss. Some filter designs may employ reduced source terminations or impedance

conversions to minimize loss. Many designs require the use of high-Q inductors and capacitors to achieve an

expected passband flatness and profile.

Figure 43. Transformer Input Network

Sampling theory states that if a signal with frequency fIN is sampled at a rate less than 2 × fIN, then it experiences

aliasing, causing the signal to fall at a new frequency between 0 and FS / 2 and become indistinguishable from

other signals at that new frequency.

To prevent out-of-band interference from aliasing onto a desired signal at a particular frequency, an anti-aliasing

filter is required at the ADC input to attenuate the interference to a level below the level of the desired signal.

This is accomplished by a lowpass filter in systems with desired signals from DC to FS / 2 or with a bandpass

filter in systems with desired signals greater than FS / 2 (under-sampled signals). If an appropriate anti-aliasing

filter is not included in the system design, the system may suffer from reduced dynamic range due to additional

noise and distortion that aliases into the frequency bandwidth of interest.

An anti-aliasing filter is required in front of the ADC input in most applications to attenuate noise and distortion at

frequencies that alias into any important frequency band of interest during the sampling process. An anti-aliasing

filter is typically a LC lowpass or bandpass filter with low insertion loss. The bandwidth of the filter is typically

designed to be less than FS / 2 to allow room for the filter transition band. Figure 44 is an example architecture of

a 9 pole order LC bandpass anti-aliasing filter with added transmission zeros that can achieve a tight filtering

profile for second Nyquist zone under-sampling applications.

Maximizing the distortion performance of this device requires the avoidance of driving circuits that are mostly

capacitive at frequencies near and above the sampling rate. The performance is maximized by ensuring the

driving circuit is high impedance or mostly resistive (real impedance) at these out-of-band frequencies. Driving

circuits with highly capacitive source impedances (negative source reactance) at these frequencies can cause

resonance with the interface, leading to sub-optimal distortion performance. In the case of bandpass LC anti-

aliasing filters, the impedance looking into the filter output is recommended to be high impedance or real at

frequencies near and above the sampling rate such as the filter shown in Figure 42 . Capacitors placed directly

at the ADC input used as bandwidth limiters or as part of a filter's final stage LC tank are not recommended.

Applications that use lumped reactive components (capacitors, inductors) in the interface to the ADC are

recommended to have a small series resistor at the ADC input, also shown in Figure 42 . Place these resistors

close to the device pins, between the external termination resistors and the device pins. A value of 5

Ω is

sufficient for most applications, though TI recommends 10

Ω for applications where the lumped differential

capacitance at the ADC input is unavoidable and greater than 2 pF.

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