Datasheet AD7716 (Analog Devices) - 9

AD7716 TERMINOLOGY OUTPUT UPDATE RATE LINEARITY ERROR
This is the rate at which the digital filter updates the output shift This is the maximum deviation of any code from a straight line register. It is a function of the master clock frequency and the passing through the endpoints of the transfer function. The programmed filter cutoff frequency. endpoints of the transfer function are zero scale (not to be con- fused with Bipolar Zero), a point 0.5 LSB below the first code
FILTER CUTOFF FREQUENCY
transition (000 . 000 to 000 . 001) and full scale, a point The digital filter of the AD7716 can be programmed, in binary 0.5 LSB above the last code transition (111 . 110 to steps, to 5 discrete cutoff frequencies, ranging from 584 Hz to 111 . 111). The error is expressed as a percentage of full 36.5 Hz (for a CLKIN frequency of 8 MHz). scale.
SETTLING TIME DIFFERENTIAL LINEARITY ERROR/NO MISSED CODES
This is the settling time of the on-chip digital filter, to 0.0007% This is the difference between any code’s actual width and the of FSR, in response to a full-scale step at the input of the ADC. ideal (1 LSB) width. Differential Linearity Error is expressed in It is proportional to the master clock frequency and the filter LSBs. A differential linearity specification of ±1 LSB or less cutoff frequency. guarantees no missed codes to the full resolution of the device. The AD7716 has no missed codes guaranteed to 21 bits with a
USABLE DYNAMIC RANGE
cutoff frequency of 146 Hz. The usable dynamic range is the ratio of the rms full-scale reading (sine wave input) to the rms noise of the converter,
GAIN ERROR
expressed in dBs. It determines the level to which it is possible Gain Error is the deviation of the last code transition to resolve the input signal. For example, at a bandwidth of (111 . 110 to 111 . 1) from the ideal (V 146 Hz, the rms noise of the converter is 11 REF –3/2 LSBs). It µV. The full-scale is expressed as a percentage of full scale. rms is 1.77 volts. So, the usable dynamic range is 104 dB. Any signal below this level will be indistinguishable from noise unless
GAIN TC
extra post-filtering techniques are employed. This is the variation of gain error with temperature and is ex- pressed in µV/°C.
TOTAL HARMONIC DISTORTION
Total harmonic distortion (THD) is the ratio of the rms sum
OFFSET ERROR
of the harmonics to the fundamental. For the AD7716, it is Offset Error is the deviation of the first code transition from the defined as: ideal (–VREF + 0.5 LSB). It is expressed as a percentage of full 2 2 2 2 2 scale. V +V +V +V +V THD (dB) = 20 log 2 3 4 5 6 V
OFFSET TC
1 This is the variation of offset error with temperature and is ex- where V1 is the rms amplitude of the fundamental and V2, V3, pressed in µV/°C. V4, V5 and V6 are the rms amplitudes of the second through sixth harmonics.
NOISE
This is the converter rms noise expressed in µV. Because of the
ABSOLUTE GROUP DELAY
digital filtering in the sigma delta converter, the noise perfor- Absolute group delay is the rate of change of phase versus fre- mance is a function of the programmed filter cutoff. quency, dφ/df and is expressed in seconds. For the AD7716, it is dependent on master clock frequency and filter cutoff
SAMPLING RATE
frequency. This is the modulator sampling rate. For the AD7716, it is fCLKIN/14.
DIFFERENTIAL GROUP DELAY
Differential group delay is the total variation in absolute group delay in the specified bandwidth. Since the digital filter in the AD7716 has perfectly linear phase, the differential group delay is almost zero. This is important in many signal processing ap- plications where excessive differential group delay can cause phase distortion. –8– REV. A