ADA2200ARUZ datasheet(12/25 Pages) AD

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Data Sheet

ADA2200

THEORY OF OPERATION

The ADA2200 is a synchronous demodulator and tunable

filter implemented with sampled analog technology (SAT).

Synchronous demodulators, also known as lock-in amplifiers,

enable accurate measurement of small ac signals in the presence

of noise interference orders of magnitude greater than the signal

amplitude. Synchronous demodulators use phase sensitive

detection to isolate the component of the signal at a specific

reference frequency and phase. Noise at frequencies that are

offset from the reference frequency are easily rejected and do

not significantly impair the measurement.

SAT works on the principle of charge sharing. A sampled

analog signal is a stepwise continuous signal without amplitude

quantization. This contrasts with a signal sampled by an ADC,

which becomes a discrete time signal with quantized amplitude.

With SAT, the input signal is sampled by holding the voltage on

a capacitor at the sampling instant. Basic signal processing can

then be performed in the analog domain by charge sharing among

capacitors. The ADA2200 includes an analog domain low-pass

decimation filter, a programmable IIR filter, and a mixer. This

combination of features enables reduced ADC sample rates and

lowers the downstream digital signal processing requirements if

the signal is digitized.

The output of the ADA2200 can also be used in an all analog

signal path. In these applications, add a reconstruction filter

following the ADA2200 in the signal path.

SYNCHRONOUS DEMODULATION BASICS

Employing synchronous demodulation as a sensor signaling

conditioning technique can result in improved sensitivity when

compared to other methods. Synchronous demodulation adds

two key benefits for recovering small sensor output signals in

the presence of noise. The first benefit being the addition of an

excitation signal, which enables the sensor output signal to be

moved to a lower noise frequency band. The second benefit is

that synchronous demodulation enables a simple low-pass filter

to remove most of the remaining undesired noise components.

Figure 16 shows a basic synchronous demodulation system used

for measuring the output of a sensor.

Figure 16. Basic Synchronous Demodulator Block Diagram

A carrier signal (fMOD) excites the sensor. This shifts the signal

generated by the physical parameter being measured by the

sensor to the carrier frequency. This shift allows the desired signal

to be placed in a frequency band with lower noise, improving

the accuracy of the measurement. A band-pass filter (BPF)

removes some of the out of band noise. A synchronous

demodulator (or mixer) shifts the signal frequency back to dc.

The last stage low-pass filter removes much of the remaining

noise. Figure 17 and Figure 18 show the frequency spectrum of

the signal at different points in the synchronous demodulator.

Figure 17. Output Spectrum of Synchronous Demodulator

Before Demodulation

Figure 18. Output Spectrum of Synchronous Demodulator

After Demodulation

Phase Sensitive Detection

Synchronous demodulation uses the principle of phase sensitive

detection to separate the signal of interest from unwanted signals.

In Figure 16, the mixer performs the phase sensitive detection.

The signal at the mixer output (C) is the product of the reference

signal and a filtered version of the sensor output (B). If the

reference signal is a sine wave, the physical parameter is a

constant and there is no noise in the system. The signal at the

output of the BPF is a sine wave that can be expressed as

VBsin(ωREFt + ϕB)

LPF

SENSOR

PHYSICAL

PARAMETER

NOISE

fMOD

fREF

BPF

fREF

NOISE AT A

SENSOR

SIGNAL AT A, B

NOISE AT B

PHYSICAL

PARAMETER

fREF

NOISE AT D

NOISE AT C

SENSOR

SIGNAL AT C, D

Rev. 0 | Page 11 of 24

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