Applications Information (Continued)
FIGURE 13. Symmetrically Centered Reference
DS005277-15
3.0 CONVERTER EQUATIONS
The transition between adjacent codes N and N + 1 is given
by:
(2)
The center of an output code N is given by:
(3)
The output code N for an arbitrary input are the integers
within the range:
(4)
where: VIN = Voltage at comparator input
VREF = Voltage at Ref(+)
VREF = Voltage at Ref(âˆ’)
VTUE = Total unadjusted error voltage (typically
VREF(+) Ã·512)
4.0 ANALOG COMPARATOR INPUTS
The dynamic comparator input current is caused by the
periodic switching of on-chip stray capacitances These are
connected alternately to the output of the resistor ladder/
switch tree network and to the comparator input as part of
the operation of the chopper stabilized comparator.
The average value of the comparator input current varies
directly with clock frequency and with VIN as shown in Figure
6.
If no filter capacitors are used at the analog or comparator
inputs and the signal source impedances are low, the com-
parator input current should not introduce converter errors,
as the transient created by the capacitance discharge will die
out before the comparator output is strobed.
If input filter capacitors are desired for noise reduction and
signal conditioning they will tend to average out the dynamic
comparator input current. It will then take on the character-
istics of a DC bias current whose effect can be predicted
conventionally. See AN-258 for further discussion.
11
www.national.com