Datasheet

LM12L458
www.ti.com SNAS085B –JULY 1999–REVISED MARCH 2013
Analog Considerations
REFERENCE VOLTAGE
The difference in the voltages applied to the V
REF+
and V
REF−
defines the analog input voltage span (the
difference between the voltages applied between two multiplexer inputs or the voltage applied to one of the
multiplexer inputs and analog ground), over which 4095 positive and 4096 negative codes exist. The voltage
sources driving V
REF+
or V
REF−
must have very low output impedance and noise.
The ADC can be used in either ratiometric or absolute reference applications. In ratiometric systems, the analog
input voltage is proportional to the voltage used for the ADC's reference voltage. When this voltage is the system
power supply, the V
REF+
pin is connected to V
A
+ and V
REF−
is connected to GND. This technique relaxes the
system reference stability requirements because the analog input voltage and the ADC reference voltage move
together. This maintains the same output code for given input conditions.
For absolute accuracy, where the analog input voltage varies between very specific voltage limits, a time and
temperature stable voltage source can be connected to the reference inputs. Typically, the reference voltage's
magnitude will require an initial adjustment to null reference voltage induced full-scale errors.
INPUT RANGE
The LM12L458's fully differential ADC and reference voltage inputs generate a two's-complement output that is
found by using the equation below.
(6)
Round up to the next integer value between −4096 to 4095 for 12-bit resolution and between −256 to 255 for 8-
bit resolution if the result of the above equation is not a whole number. As an example, V
REF+
= 2.5V, V
REF−
= 1V,
V
IN+
= 1.5V and V
IN−
= GND. The 12-bit + sign output code is positive full-scale, or 0,1111,1111,1111. If V
REF+
=
3.3V, V
REF−
= 1V, V
IN+
= 3V, and V
IN−
= GND, the 12-bit + sign output code is 0,1100,0000,0000.
INPUT CURRENT
A charging current flows into or out of (depending on the input voltage polarity) the analog input pins, IN0–IN7 at
the start of the analog input acquisition time (t
ACQ
). This current's peak value will depend on the actual input
voltage applied. This charging current causes voltage spikes at the inputs. This voltage spikes will not corrupt the
conversion results.
INPUT SOURCE RESISTANCE
For low impedance voltage sources (<80Ω for 6 MHz operation) the input charging current will decay, before the
end of the S/H's acquisition time, to a value that will not introduce any conversion errors. For higher source
impedances, the S/H's acquisition time can be increased. As an example, operating with a 6 MHz clock
frequency and maximum acquisition time, the LM12L458's analog inputs can handle source impedance as high
as 5.56 kΩ. Refer to Section 2.1, Instruction RAM “00”, Bits 12–15 for further information.
INPUT BYPASS CAPACITANCE
External capacitors (0.01 μF to 0.1 μF) can be connected between the analog input pins, IN0–IN7, and analog
ground to filter any noise caused by inductive pickup associated with long input leads. It will not degrade the
conversion accuracy.
NOISE
The leads to each of the analog multiplexer input pins should be kept as short as possible. This will minimize
input noise and clock frequency coupling that can cause conversion errors. Input filtering can be used to reduce
the effects of the noise sources.
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