TC500
TC500A
TC510
TC514
Table 1. Internal Analog Gate Status
PRECISION ANALOG FRONT ENDS
Internal Analog Gate Status
Conversion Phase
Auto-Zero (A = 0, B = 1)
Input Signal Integration
(A = 1, B = 0)
Reference Voltage
Deintegration (A =1, B= 1)
Integrator Output Zero
(A = 0, B = 0)
SW
I
Closed
+
SW
RI
–
SW
RI
SW
Z
Closed
SW
R
Closed
SW
1
Closed
SW
IZ
Closed*
Closed
Closed
Closed
Closed
–
*Assumes a positive polarity input signal. SW
RI
would be closed for a negative input signal.
Analog Input Signal Integration Phase
(INT)
The TC5xx integrates the differential voltage between
+
–
the (V
IN
) and (V
IN
) inputs. The differential voltage must be
within the device's common-mode range V
CMR
.
The input signal polarity is normally checked via soft-
ware at the end of this phase: CMPTR = 1 for positive
polarity; CMPTR = 0 for negative polarity.
voltage range, common-mode rejection is typically 80dB.
Full accuracy is maintained, however, when the inputs are
no less than 1.5V from either supply.
The integrator output also follows the common-mode
voltage. The integrator output must not be allowed to satu-
rate. A worst-case condition exists, for example, when a
large, positive common-mode voltage with a near full-scale
negative differential input voltage is applied. The negative
input signal drives the integrator positive when most of its
swing has been used up by the positive common-mode
voltage. For these critical applications, the integrator swing
can be reduced. The integrator output can swing within 0.9V
of either supply without loss of linearity.
Reference Voltage Deintegration Phase
(DINT)
The previously charged reference capacitor is con-
nected with the proper polarity to ramp the integrator output
back to zero. An externally-provided, precision timer is used
to measure the duration of this phase. The resulting time
measurement is proportional to the magnitude of the applied
input voltage.
Analog Common
Analog common is used as V
IN
return during system-
–
zero and reference deintegrate. If V
IN
is different from analog
common, a common-mode voltage exists in the system.
This signal is rejected by the excellent CMR of the converter.
–
In most applications, V
IN
will be set at a fixed known voltage
(i.e., power supply common). A common-mode voltage will
–
exist when V
IN
is not connected to analog common.
–
Differential Reference
(V
+
, V
REF
)
REF
Integrator Output Zero Phase
(IZ)
This phase guarantees the integrator output is at 0V
when the Auto Zero phase is entered and that only system
offset voltages are compensated. This phase is used at the
end of the reference voltage deintegration phase and MUST
be used for ALL TC5xx applications having resolutions of 12
bits or more. If this phase is not used, the value of the Auto-
Zero capacitor (C
AZ
) must be about 2 to 3 times the value of
the integration capacitor (C
INT
) to reduce the effects of
charge-sharing. The Integrator Output Zero phase should
be programmed to operate until the Output of the Compara-
tor returns "HIGH". The overall Timing System is shown in
Figure 8.
ANALOG SECTION
+
–
Differential Inputs
(V
IN
, V
IN
)
The TC5xx operates with differential voltages within the
input amplifier common-mode range. The amplifier com-
mon-mode range extends from 1.5V below positive supply
to 1.5V above negative supply. Within this common-mode
3-26
The reference voltage can be anywhere within 1V of the
power supply voltage of the converter. Roll-over error is
caused by the reference capacitor losing or gaining charge
due to stray capacitance on its nodes. The difference in
reference for (+) or (–) input voltages will cause a roll-over
error. This error can be minimized by using a large reference
capacitor in comparison to the stray capacitance.
Phase Control Inputs
(A, B)
The A, B unlatched logic inputs select the TC5xx oper-
ating phase. The A, B inputs are normally driven by a
microprocessor I/O port or external logic.
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