Datasheet AD7898 (Analog Devices) - 7

AD7898 TERMINOLOGY Relative Accuracy Signal to (Noise + Distortion) Ratio
Relative accuracy or endpoint nonlinearity is the maximum This is the measured ratio of signal to (noise + distortion) at the deviation from a straight line passing through the endpoints of output of the A/D converter. The signal is the rms amplitude of the ADC transfer function. the fundamental. Noise is the rms sum of all nonfundamental
Differential Nonlinearity
signals up to half the sampling frequency (fS/2), excluding dc. This is the difference between the measured and the ideal The ratio is dependent upon the number of quantization levels 1 LSB change between any two adjacent codes in the ADC. in the digitization process; the more levels, the smaller the quan- tization noise. The theoretical signal to (noise + distortion) ratio
Positive Full-Scale Error (AD7898-10)
for an ideal N-bit converter with a sine wave input is given by: This is the deviation of the last code transition (01 . 110 to 01 . 111) from the ideal (4 × VREF – 3/2 LSB) after the Signal to (Noise + Distortion) = (6.02 N + 1.76) dB Bipolar Zero Error has been adjusted out. Thus for a 12-bit converter, this is 74 dB.
Positive Full-Scale Error (AD7898-3) Total Harmonic Distortion
This is the deviation of the last code transition (01 . 110 to Total harmonic distortion (THD) is the ratio of the rms sum of 01 . 111) from the ideal (VREF – 3/2 LSB) after the Bipolar harmonics to the fundamental. For the AD7898, it is defined as: Zero Error has been adjusted out.
Bipolar Zero Error (AD7898-10, AD7898-3)
2 2 2 2 2 V2 + V3 + V4 + V5 + V This is the deviation of the midscale transition (all 0s to all 1s) THD (dB )= 20 log 6 from the ideal AGND – 1/2 LSB. V1
Negative Full-Scale Error (AD7898-10)
where V1 is the rms amplitude of the fundamental, and V2, V3, This is the deviation of the first code transition (10 . 000 to V4, V5, and V6 are the rms amplitudes of the second through the 10 . 001) from the ideal (–4 × VREF + 1/2 LSB) after Bipo- sixth harmonics. lar Zero Error has been adjusted out.
Peak Harmonic or Spurious Noise Negative Full-Scale Error (AD7898-3)
Peak harmonic or spurious noise is defined as the ratio of the This is the deviation of the first code transition (10 . 000 to rms value of the next largest component in the ADC output 10 . 001) from the ideal (–VREF + 1/2 LSB) after Bipolar spectrum (up to fS/2 and excluding dc) to the rms value of the Zero Error has been adjusted out. fundamental. Normally, the value of this specification is deter- mined by the largest harmonic in the spectrum, but for parts
Track/Hold Acquisition Time
where the harmonics are buried in the noise floor, it will be a Track/Hold acquisition time is the time required for the output noise peak. of the track/hold amplifier to reach its final value, within ± 1/2 LSB, after the end of conversion (the point at which the
Intermodulation Distortion
track/hold returns to track mode). It also applies to situations With inputs consisting of sine waves at two frequencies, fa and where there is a step input change on the input voltage applied fb, any active device with nonlinearities will create distortion to the V products at sum and difference frequencies of mfa ± nfb where IN input of the AD7898. This means that the user must wait for the duration of the track/hold acquisition time after the m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for which end of conversion, or after a step input change to V neither m nor n are equal to zero. For example, the second order IN, before starting another conversion to ensure that the part operates to terms include (fa + fb) and (fa – fb), while the third order terms specification. include (2 fa + fb), (2 fa – fb), (fa + 2 fb) and (fa – 2 fb).
PSR (Power Supply Rejection)
The AD7898 is tested using the CCIF standard where two input Variations in power supply will affect the full-scale transition, frequencies are used. In this case, the second and third order but not the converter’s linearity. Power Supply Rejection is the terms are of different significance. The second order terms are maximum change in full-scale transition point due to change in usually distanced in frequency from the original sine waves, power-supply voltage from the nominal value. while the third order terms are usually at a frequency close to the input frequencies. As a result, the second and third order terms are specified separately. The calculation of the intermodulation distortion is as per the THD specification where it is the ratio of the rms sum of the individual distortion products to the rms amplitude of the fundamental expressed in dBs. –6– REV. A Document Outline FEATURES GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM PRODUCT HIGHLIGHTS SPECIFICATIONS TIMING SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ORDERING GUIDE PIN CONFIGURATION PIN FUNCTION DESCRIPTIONS TERMINOLOGY Signal to (Noise + Distortion) Ratio Total Harmonic Distortion Peak Harmonic or Spurious Noise Intermodulation Distortion Relative Accuracy Differential Nonlinearity Positive Full-Scale Error (AD7898-10) Positive Full-Scale Error (AD7898-3) Bipolar Zero Error (AD7898-10, AD7898-3) Negative Full-Scale Error (AD7898-10) Negative Full-Scale Error (AD7898-3) Track/Hold Acquisition Time PSR (Power Supply Rejection) PERFORMANCE CURVES Noise CONVERTER DETAILS CIRCUIT DESCRIPTION Analog Input Section Acquisition Time TYPICAL CONNECTION DIAGRAM VDRIVE Feature Track/Hold Section Reference Input SERIAL INTERFACE OPERATING MODES Mode 0 Operation Mode 1 Operation Mode Selection Power-Down Mode Power-Up Times MICROPROCESSOR/MICROCONTROLLER INTERFACE FOR MODE 0 OPERATION 8x51/L51 to AD7898 Interface 68HC11/L11 to AD7898 Interface ADSP-2103/ADSP-2105 to AD7898 Interface DSP56002/L002 to AD7898 Interface MICROPROCESSOR INTERFACING FOR MODE 1 TMS320C5x/C54x to AD7898 Interface AD7898 to ADSP-21xx Interface AD7898 to DSP56xxx Interface AD7898 to MC68HC16 Interface OUTLINE DIMENSIONS Revision History