# Analyzing Simple ADC with Impairments

This example shows how to implement a basic ADC using a Zero-Order Hold block as a sampler. This simple ADC highlights some of the typical impairments introduced in an analog-to-digital converters such as aperture jitter, nonlinearity, quantization, and saturation. This example shows how to measure the effects of such impairments using a Spectrum Analyzer block and the ADC AC Measurement block from the Mixed-Signal Blockset™. To better approximate real-world performance, you can individually enable the impairments in the model.

```
model = 'MSADCImpairments';
open_system(model)
```

To observe the behavior of an ideal ADC, bypass the impairments using the switches. Set the Sine Wave source to generate two tones as an input signal.

set_param([model '/Aperture Jitter'],'sw','1'); set_param([model '/Non Linearity'],'sw','0'); set_param([model '/Quantization and Saturation'],'sw','0'); set_param([model '/Sine Wave'],'Frequency', '2*pi*[47 53]*1e6');

Simulate the model and observe the expected clean output spectrum of the ADC.

sim(model);

### Effect of Aperture Jitter

Set the first switch to the down position. The Variable Delay block delays the signal sample-by-sample by the amount on its *td* input. The Noise Source block generates a uniform random variable, which is low-pass filtered by the Shape the jitter noise spectrum block before it arrives at the *td* input to the Variable Delay. Use a shaped uniform noise distribution to represent the jitter. Notice that in this model, the clock of the ADC is specified in the ideal zero-order hold block, and it is equal to 1/Fs, where Fs is a MATLAB® variable defined in the model initialization callback and equal to `1.024`

GHz.

set_param([model '/Aperture Jitter'],'sw','0');

As expected, the spectrum degrades because of the presence of the jitter.

sim(model);

### Effect of Nonlinearity

Set the second switch to the up position. This enables the ADC nonlinearity. A scaled hyperbolic tangent function provides nonlinearity. Its scale factor, *alpha*, determines the amount of nonlinearity the tanh applies to the signal. By default, *alpha* is `0.01`

.

set_param([model '/Non Linearity'],'sw','1');

The spectrum degrades because of the nonlinearity as higher order harmonics get generated.

sim(model);

### Effect of Quantization and Saturation

Set the third switch to the up position enabling the ADC quantization and hard saturation.

set_param([model '/Quantization and Saturation'],'sw','1');

The spectrum degrades because of the quantization effects. The noise floor raises as seen in the spectrum.

sim(model);

### ADC AC Measurements

Use the ADC AC Measurement block in the Mixed-Signal Blockset™ to measure the noise performance of the ADC and compute the effective number of bits (ENOB).

Use single sinusoidal tone as input to the ADC to measure other metrics.

```
bdclose(model);
model = 'MSADCImpairments_AC';
open_system(model);
```

Ftest = 33/round(2*pi*2^8)*Fs; set_param([model '/Sine Wave'],'Frequency', '2*pi*Ftest'); scopecfg = get_param([model '/Spectrum Scope'], 'ScopeConfiguration'); scopecfg.DistortionMeasurements.Algorithm = 'Harmonic'; scopecfg.FFTLength = '512'; scopecfg.WindowLength = '512'; sim(model);

The Aperture Jitter Measurement block from Mixed-Signal Blockset™ measures the average jitter introduced on the signal to be approximately equal to `1`

ps.

Additionally, use the spectrum analyzer to measure:

Output Third Order Intercept Point (OIP3)

Signal to Noise Ratio (SNR)

Total Harmonic Distortion (THD)

Increase the factor *alpha* to increase the nonlinearity of the ADC and make the effects of nonlinearity more evident on top of the noise floor. This is just for demonstration purposes.

alpha = 0.8;

Use a two tone test signal as input to the ADC for the intermodulation measurements.

set_param([model '/Sine Wave'],'Frequency', '2*pi*[50e6 75e6]');

To enable distortion measurements in the spectrum analyzer, click on **Distortion Measurement** as in the figure below and select `Intermodulation`

as **Distortion** type.

scopecfg.DistortionMeasurements.Algorithm = 'Intermodulation'; scopecfg.FFTLength = '4096'; scopecfg.WindowLength = '4096'; sim(model);

The scope allows for the measurement of the third order products adjacent to the input signals, and determines the output referred third order intercept point.

## See Also

Sampling Clock Source | Aperture Jitter Measurement | ADC AC Measurement