Datasheet

AD7740
REV. A
–5–
TERMINOLOGY
INTEGRAL NONLINEARITY
For the VFC, Integral Nonlinearity (INL) is a measure of the maxi-
mum deviation from a straight line passing through the actual
endpoints of the VFC transfer function. The error is expressed in
% of the actual frequency span:
Frequency Span = FOUT(max) – FOUT(min)
OFFSET ERROR
Ideally, the output frequency for 0 V input voltage is 10% of
f
CLKIN
in unbuffered mode. The deviation from this value referred
to the input is the offset error at BUF = 0. In buffered mode the
minimum output frequency (corresponding to 0.10 V minimum
input voltage) is 13.2% of f
CLKIN
at V
REF
= 2.5 V. The deviation
from this value referred to the input is the offset error at BUF = 1.
Offset error is expressed in mV.
GAIN ERROR
This is a measure of the span error of the VFC. The gain is the
scale factor that relates the input VIN to the output FOUT.
The gain error is the deviation in slope of the actual VFC transfer
characteristic from the ideal expressed as a percentage of the full-
scale span. See Figure 2.
OFFSET ERROR DRIFT
This is a measure of the change in Offset Error with changes in
temperature. It is expressed in µV/°C.
GAIN ERROR DRIFT
This is a measure of the change in Gain Error with changes in
temperature. It is expressed in (ppm of span)/°C.
POWER SUPPLY REJECTION RATIO (PSRR)
This indicates how the apparent input voltage of the VFC is
affected by changes in the supply voltage. The input voltage is
kept constant at 2 V, V
REF
is 2.5 V and the VDD supply is varied
⫾10% at 3.3 V and ±5% at 5 V. The ratio of the apparent change
in input voltage to the change in VDD is measured in dBs.
0
REFIN
WITH OFFSET
ERROR ONLY
IDEAL
GAIN ERROR
OUTPUT
FREQUENCY
FOUT
0.9 f
CLKIN
OFFSET
ERROR
WITH OFFSET
ERROR AND
GAIN ERROR
INPUT
VOLTAGE
VIN
0.1
f
CLKIN
Figure 2. Offset and Gain