Datasheet AD7891 (Analog Devices) - 10

AD7891 TERMINOLOGY Channel-to-Channel Isolation Signal-to-(Noise + Distortion) Ratio
Channel-to-channel isolation is a measure of the level of This is the measured ratio of signal to (noise + distortion) at the crosstalk between channels. It is measured by applying a full- output of the ADC. The signal is the rms amplitude of the scale 20 kHz (AD7891-1) or 100 kHz (AD7891-2) sine wave fundamental. Noise is the rms sum of all nonfundamental signal to one input channel and determining how much that signals up to half the sampling frequency (fS/2), excluding dc. signal is attenuated in each of the other channels. The figure The ratio is dependent upon the number of quantization levels given is the worst case across all eight channels. in the digitization process; the more levels, the smaller the quan-
Relative Accuracy
tization noise. The theoretical signal-to-(noise +distortion) ratio Relative accuracy or endpoint nonlinearity is the maximum for an ideal N-bit converter with a sine wave input is given by deviation from a straight line passing through the endpoints of Signal-to-(Noise + Distortion) = (6.02N + 1.76) dB the ADC transfer function. Therefore, for a 12-bit converter, this is 74 dB.
Differential Nonlinearity
This is the difference between the measured and the ideal 1 LSB
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of harmonics to the change between any two adjacent codes in the ADC. fundamental. For the AD7891, it is defined as
Positive Full-Scale Error (AD7891-1,
ⴞ
10 V and
ⴞ
5 V; AD7891-2,
ⴞ
2.5 V)
2 2 2 2 2 V V V V V This is the deviation of the last code transition (01. .110 to THD dB log ( ) = 2 + 3 + 4 + 5 + 20 6 01. .111) from the ideal 4 ¥ REF IN – 3/2 LSB (AD7891-1 V1 ±10 V range), 2 ¥ REF IN – 3/2 LSB (AD7891-1 ± 5 V range), where V or REF IN – 3/2 LSB (AD7891-2, ± 2.5 V range), after the 1 is the rms amplitude of the fundamental and V2, V3, V bipolar zero error has been adjusted out. 4, V5 and V6 are the rms amplitudes of the second through the sixth harmonics.
Positive Full-Scale Error (AD7891-2, 0 V to 5 V and 0 V to 2.5 V) Peak Harmonic or Spurious Noise
This is the deviation of the last code transition (11. .110 to Peak harmonic or spurious noise is defined as the ratio of the rms 11. .111) from the ideal 2 ¥ REF IN – 3/2 LSB (0 V to 5 V value of the next largest component in the ADC output spectrum range), or REF IN – 3/2 LSB (0 V to 2.5 V range), after the (up to f unipolar offset error has been adjusted out. S/2 and excluding dc) to the rms value of the fundamental. Normally, the value of this specification is determined by the larg-
Bipolar Zero Error (AD7891-1,
ⴞ
10 V and
ⴞ
5 V; AD7891-2,
ⴞ
2.5 V)
est harmonic in the spectrum, but for parts where the harmonics This is the deviation of the midscale transition (all 0s to all 1s) are buried in the noise floor, it is a noise peak. from the ideal AGND – 1/2 LSB.
Intermodulation Distortion Unipolar Offset Error (AD7891-2, 0 V to 5 V and 0 V to 2.5 V)
With inputs consisting of sine waves at two frequencies, fa and This is the deviation of the first code transition (00. .000 to fb, any active device with nonlinearities creates distortion 00. .001) from the ideal AGND + 1/2 LSB. products at sum and difference frequencies of mfa ± nfb, where m, n = 0, 1, 2, 3, and so on. Intermodulation terms are those for
Negative Full-Scale Error (AD7891-1,
ⴞ
10 V and
ⴞ
5 V;
which neither m nor n are equal to zero. For example, the
AD7891-2,
ⴞ
2.5 V)
second-order terms include (fa + fb) and (fa – fb), while the This is the deviation of the first code transition (10. .000 to third-order terms include (2fa + fb), (2fa – fb), (fa + 2fb), and 10. .001) from the ideal –4 ¥ REF IN + 1/2 LSB (AD7891-1 ± (fa – 2fb). 10 V range), –2 ¥ REF IN + 1/2 LSB (AD7891-1 ± 5 V range), or –REF IN + 1/2 LSB (AD7891-2, ± 2.5 V range), after bipolar The AD7891 is tested using the CCIF standard where two zero error has been adjusted out. input frequencies near the top end of the input bandwidth are used. In this case, the second- and third-order terms are of
Track/Hold Acquisition Time
different significance. The second-order terms are usually dis- Track/hold acquisition time is the time required for the output of tanced in frequency from the original sine waves while the third- the track/hold amplifier to reach its final value, within ±1/2 LSB, order terms are usually at a frequency close to the input after the end of conversion (the point at which the track/hold frequencies. As a result, the second- and-third order terms are returns to track mode). It also applies to situations where a specified separately. The calculation of the intermodulation change in the selected input channel takes place or where there distortion is as per the THD specification where it is the ratio of is a step input change on the input voltage applied to the selected the rms sum of the individual distortion products to the rms VIN input of the AD7891. It means the user must wait for the amplitude of the fundamental expressed in dBs. duration of the track/hold acquisition time after the end of conversion or after a channel change/step input change to VIN before starting another conversion, to ensure the part operates to specification. REV. D –9– Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM PRODUCT HIGHLIGHTS SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS TIMING CHARACTERISTICS ORDERING GUIDE PIN CONFIGURATIONS PIN FUNCTION DESCRIPTIONS PARALLEL INTERFACE MODE FUNCTIONS Data I/O Lines Parallel Read Operation SERIAL INTERFACE MODE FUNCTIONS CONTROL REGISTER TERMINOLOGY Signal-to-(Noise + Distortion) Ratio Total Harmonic Distortion (THD) Peak Harmonic or Spurious Noise Intermodulation Distortion Channel-to-Channel Isolation Relative Accuracy Differential Nonlinearity Positive Full-Scale Error (AD7891-1, ±10 V and ±5 V; AD7891-2, ±2.5 V) Positive Full-Scale Error (AD7891-2, 0 V to 5 V and 0 V to 2.5 V) Bipolar Zero Error (AD7891-1, ±10 V and ±5 V; AD7891-2, ±2.5 V) Unipolar Offset Error (AD7891-2, 0 V to 5 V and 0 V to 2.5 V) Negative Full-Scale Error (AD7891-1, ±10 V and ±5V; AD7891-2, ±2.5 V) Track/Hold Acquisition Time CONVERTER DETAILS INTERFACE INFORMATION Parallel Interface Mode Serial Interface Mode Simplifying the Serial Interface CIRCUIT DESCRIPTION Reference Analog Input Section Track/Hold Amplifier STANDBY Operation MICROPROCESSOR INTERFACING AD7891 to 8X51 Serial Interface AD7891 to 68HC11 Serial Interface AD7891 to ADSP-21xx Serial Interface AD7891 to DSP5600x Serial Interface AD7891 to TMS320xxx Serial Interface PARALLEL INTERFACING AD7891 to ADSP-21xx AD7891 to TMS32020, TMS320C25, and TMS320C5x AD7891 to TMS320C3x AD7891 to DSP5600x Power Supply Bypassing and Grounding AD7891 PERFORMANCE Linearity Noise Dynamic Performance Effective Number of Bits OUTLINE DIMENSIONS Revision History