2

SIGNAL PATH

designer

Selecting Amplifiers, ADCs, and Clocks for High-Performance Signal Paths

Ignoring the filter, the noise of the overall cascaded

path shown is given by Frii’s equation:

The noise of the ADC driver is divided by the gain

of the LNA and consequently, it is best to select the

lowest-noise LNA available and take as much gain

as possible at this first stage. Since the noise of the

driver is divided by the LNA gain, it becomes less

critical to the overall noise performance. In fact, the

further along the signal path, the less critical the

noise performance of each stage becomes.

The building block after the LNA is the ADC-driver

stage. In a system that responds down to signals at

0 Hz, a DC-coupled amplifier is the only choice,

while in an AC-coupled system, a transformer can

also be used. However, transformers are limited in

their frequency range of operation and can have poor

differential output balance, which is important when

driving

differential-input

ADCs.

When providing gain, transformers

also multiply the source impedance

driving the ADC by the transformer

turns ratio squared. This reduces

the pole frequency formed with the

ADC-input

capacitance,

thereby

reducing system bandwidth. Even

though amplifiers can add more

noise than a transformer, they have

better gain flatness and can provide

a range of desired gains by setting

external resistors. The gain of a trans-

former is limited by achievable turns

ratios. Amplifiers have lower output

impedance which is not significantly

affected by the choice of gain.

The signal path between each stage may be single-

ended or differential, depending on the initial

signal source. For a source with a single-ended

output, a “single-to-diff stage” can be used to create

differential-drive signals. Differential signal paths

are higher performance, but the drawbacks include

an increase in the number of components, board

area, cost, and complexity of the filter.

Types of Data Acquisition Systems

Sampled-data systems can be split into two main

types. The simplest is the baseband system also

known as the “1st-Nyquist-zone” system. The second

is the more complex under-sampled system, often

referred to as bandpass, narrow band, sub-sampled, or

Intermediate

Frequency

(IF)-sampled

system.

Baseband-system signal paths are generally DC-

coupled while IF-bandpass signal paths tend to be

AC-coupled. In a conventional 1st-Nyquist-zone

system, the ADC samples the input at sample rate,

back down into the 1st Nyquist zone as shown in

Figure 2b, the ADC input is normally band-limited

to the 1st Nyquist zone by a low-pass channel filter.

Frequency

1st Nyquist Zone

2nd Nyquist Zone

3rd Nyquist Zone

4th Nyquist Zone

Input

Signal

Input

Image

Input

Image

Input

Image

Input

Image

5th Nyquist Zone

Wanted signal

band

ADC Dynamic

Range

1st Nyquist Zone

2nd Nyquist Zone

3rd Nyquist Zone

4th Nyquist Zone

Frequency

Input

Signal

Input

Image

Input

Image

5th Nyquist Zone

Input Signal ‘Aliased’

by Spur Image

Unwanted Input

Signal Spur

Input

Image

Input

Image

Spur

Image

Spur

Image

Spur

Image

Wanted signal

band

ADC Dynamic

Range

+

=

+

Figure 2a. 1st Nyquist baseband sampling where (fs>2fH)

Figure 2b. 1st Nyquist sampling with no ADC input filter showing input

spur >fs/2 aliasing back into 1st Nyquist zone to interfere with input< fs/2

SignalPathDesigner.indd 2

SignalPathDesigner.indd 2

9/5/07 3:24:30 PM

9/5/07 3:24:30 PM