REV. A
AD7716
–8–
OUTPUT UPDATE RATE
This is the rate at which the digital filter updates the output shift
register. It is a function of the master clock frequency and the
programmed filter cutoff frequency.
FILTER CUTOFF FREQUENCY
The digital filter of the AD7716 can be programmed, in binary
steps, to 5 discrete cutoff frequencies, ranging from 584 Hz to
36.5 Hz (for a CLKIN frequency of 8 MHz).
SETTLING TIME
This is the settling time of the on-chip digital filter, to 0.0007%
of FSR, in response to a full-scale step at the input of the ADC.
It is proportional to the master clock frequency and the filter
cutoff frequency.
USABLE DYNAMIC RANGE
The usable dynamic range is the ratio of the rms full-scale
reading (sine wave input) to the rms noise of the converter,
expressed in dBs. It determines the level to which it is possible
to resolve the input signal. For example, at a bandwidth of
146 Hz, the rms noise of the converter is 11 µV. The 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
extra post-filtering techniques are employed.
TOTAL HARMONIC DISTORTION
Total harmonic distortion (THD) is the ratio of the rms sum
of the harmonics to the fundamental. For the AD7716, it is
defined as:
THD (dB) = 20 log
V
2
2
+V
3
2
+V
4
2
+V
5
2
+V
6
2
V
1
where V
1
is the rms amplitude of the fundamental and V
2
, V
3
,
V
4
, V
5
and V
6
are the rms amplitudes of the second through
sixth harmonics.
ABSOLUTE GROUP DELAY
Absolute group delay is the rate of change of phase versus fre-
quency, dφ/df and is expressed in seconds. For the AD7716,
it is dependent on master clock frequency and filter cutoff
frequency.
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.
TERMINOLOGY
LINEARITY ERROR
This is the maximum deviation of any code from a straight line
passing through the endpoints of the transfer function. The
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
transition (000 . . . 000 to 000 . . . 001) and full scale, a point
0.5 LSB above the last code transition (111 . . . 110 to
111 . . . 111). The error is expressed as a percentage of full
scale.
DIFFERENTIAL LINEARITY ERROR/NO MISSED CODES
This is the difference between any code’s actual width and the
ideal (1 LSB) width. Differential Linearity Error is expressed in
LSBs. A differential linearity specification of ±1 LSB or less
guarantees no missed codes to the full resolution of the device.
The AD7716 has no missed codes guaranteed to 21 bits with a
cutoff frequency of 146 Hz.
GAIN ERROR
Gain Error is the deviation of the last code transition
(111 . . . 110 to 111 . . . 1) from the ideal (V
REF
–3/2 LSBs). It
is expressed as a percentage of full scale.
GAIN TC
This is the variation of gain error with temperature and is ex-
pressed in µV/°C.
OFFSET ERROR
Offset Error is the deviation of the first code transition from the
ideal (–V
REF
+ 0.5 LSB). It is expressed as a percentage of full
scale.
OFFSET TC
This is the variation of offset error with temperature and is ex-
pressed in µV/°C.
NOISE
This is the converter rms noise expressed in µV. Because of the
digital filtering in the sigma delta converter, the noise perfor-
mance is a function of the programmed filter cutoff.
SAMPLING RATE
This is the modulator sampling rate. For the AD7716, it is
f
CLKIN
/14.
