MAX1021 datasheet(41/47 Pages) MAXIM

41 / 47 page

10-Bit, Multichannel ADCs/DACs with FIFO,

Temperature Sensing, and GPIO Ports

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Unipolar ADC Offset Error

For an ideal converter, the first transition occurs at 0.5

LSB, above zero. Offset error is the amount of deviation

between the measured first transition point and the

ideal first transition point.

Bipolar ADC Offset Error

While in bipolar mode, the ADC’s ideal midscale transi-

tion occurs at AGND -0.5 LSB. Bipolar offset error is the

measured deviation from this ideal value.

ADC Gain Error

Gain error is defined as the amount of deviation

between the ideal transfer function and the measured

transfer function, with the offset error removed and with

a full-scale analog input voltage applied to the ADC,

resulting in all ones at DOUT.

DAC Offset Error

DAC offset error is determined by loading a code of all

zeros into the DAC and measuring the analog output

voltage.

DAC Gain Error

DAC gain error is defined as the amount of deviation

between the ideal transfer function and the measured

transfer function, with the offset error removed, when

loading a code of all ones into the DAC.

Aperture Jitter

Aperture jitter (tAJ) is the sample-to-sample variation in

the time between the samples.

Aperture Delay

Aperture delay (tAD) is the time between the rising

edge of the sampling clock and the instant when an

actual sample is taken.

Signal-to-Noise Ratio

For a waveform perfectly reconstructed from digital sam-

ples, signal-to-noise ratio (SNR) is the ratio of full-scale

analog input (RMS value) to the RMS quantization error

(residual error). The ideal, theoretical minimum analog-

to-digital noise is caused by quantization error only and

results directly from the ADC’s resolution (N bits):

SNR = (6.02 x N + 1.76)dB

In reality, there are other noise sources besides quanti-

zation noise, including thermal noise, reference noise,

clock jitter, etc. Therefore, SNR is calculated by taking

the ratio of the RMS signal to the RMS noise. RMS noise

includes all spectral components to the Nyquist fre-

quency excluding the fundamental, the first five har-

monics, and the DC offset.

Signal-to-Noise Plus Distortion

Signal-to-noise plus distortion (SINAD) is the ratio of the

fundamental input frequency’s RMS amplitude to the

RMS equivalent of all other ADC output signals:

SINAD(dB) = 20 x log (SignalRMS / NoiseRMS)

Effective Number of Bits

Effective number of bits (ENOB) indicates the global

accuracy of an ADC at a specific input frequency and

sampling rate. An ideal ADC’s error consists of quanti-

zation noise only. With an input range equal to the full-

scale range of the ADC, calculate the ENOB as follows:

ENOB = (SINAD - 1.76) / 6.02

Total Harmonic Distortion

Total harmonic distortion (THD) is the ratio of the RMS

sum of the first five harmonics of the input signal to the

fundamental itself. This is expressed as:

where V1 is the fundamental amplitude, and V2 through

V6 are the amplitudes of the first five harmonics.

Spurious-Free Dynamic Range

Spurious-free dynamic range (SFDR) is the ratio of RMS

amplitude of the fundamental (maximum signal compo-

nent) to the RMS value of the next largest distortion

component.

ADC Channel-to-Channel Crosstalk

Bias the ON channel to midscale. Apply a full-scale sine

wave test tone to all OFF channels. Perform an FFT on

the ON channel. ADC channel-to-channel crosstalk is

expressed in dB as the amplitude of the FFT spur at the

frequency associated with the OFF channel test tone.

Intermodulation Distortion (IMD)

IMD is the total power of the intermodulation products

relative to the total input power when two tones, f1 and

f2, are present at the inputs. The intermodulation prod-

ucts are (f1 ± f2), (2 x f1), (2 x f2), (2 x f1 ± f2), (2 x f2 ±

f1). The individual input tone levels are at -7dB FS.

Small-Signal Bandwidth

A small -20dB FS analog input signal is applied to an

ADC so the signal’s slew rate does not limit the ADC’s

performance. The input frequency is then swept up to

the point where the amplitude of the digitized conver-

sion result has decreased by -3dB. Note that the T/H

performance is usually the limiting factor for the small-

signal input bandwidth.

THD

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