PRECISION ANALOG FRONT ENDS
TC500
TC500A
TC510
1
2
3
4
5
6
7
8
TC514
S
S
S
30 µV
NTH
NTH
NTH
Normal
LowREF
High
VREF
VREF
VREF
RINT CINT
Figure 6. Noise
SLOPE (S) =
NTH = Noise Threshold
Figure 8 shows the overall timing for a typical system in
which a TC5xx is interfaced to a microcontroller. The
microcontroller drives the A, B inputs with I/O lines and
monitors the comparator output, CMPTR, using an I/O line
or dedicated timer-capture control pin. It may be necessary
to monitor the state of the CMPTR output in addition to
havingitcontrolatimerdirectlyfortheReferenceDeintegra-
tion phase. (This is further explained below.)
The timing diagram in Figure 8 is not to scale as the
timinginarealsystemdependsonmanysystemparameters
and component value selections. There are four critical
timing events (as shown in Figure 8): sampling the input
polarity; capturing the deintegration time; minimizing over-
shoot and properly executing the Integrator Output Zero
phase.
INTEGRATOR
OUTPUT
ZERO
CROSSING
OVERSHOOT
COMPARATOR
OUTPUT COMP
DEINTEGRATE PHASE
INTEGRATOR
ZERO PHASE
INTEGRATE
PHASE
Figure 7. Overshoot
Auto-Zero Phase
The length of this phase is usually set to be equal to the
Input Signal Integration time. This decision is virtually arbi-
trary since the magnitudes of the various system errors are
not known. Setting the Auto-Zero time equal to the Input
Integrate time should be more than adequate to null out
system errors. The system may remain in this phase indefi-
nitely,i.e.,Auto-ZeroistheappropriateidlestateforaTC5xx
device.
DESIGN CONSIDERATIONS
Noise
The threshold noise (NTH) is the algebraic sum of the
integrator noise and the comparator noise. This value is
typically 30µV. Figure 6 shows how the value of the refer-
ence voltage can affect the final count. Such errors can be
reduced by increased integration times, in the same way
that 50/60Hz noise is rejected. The signal-to-noise ratio is
related to the integration time (tINT) and the integration time
constant (RINT) (CINT) as follows:
Input Signal Integrate Phase
Thelengthofthisphaseisconstantfromoneconversion
to the next and depends on system parameters and compo-
nent value selections. The calculation of tINT is shown
elsewhere in this data sheet. At some point near the end of
this phase, the microcontroller should sample CMPTR to
determine the input signal polarity. This value is, in effect,
the Sign Bit for the overall conversion result. Optimally,
CMPTR should be sampled just before this phase is termi-
nated by changing AB from 10 to 11. The consideration here
VIN
tINT
S/N (dB) = 20 Log
•
(
)
30 x 10–6
(RINT) • (CINT
)
System Timing
To obtain maximum performance from the TC5xx, the
overshoot at the end of the Deintegration phase must be
minimized.Also,theAutoZerophasemustbeterminatedas
soon as the comparator output returns high. (See timing
diagram, Figure 8).
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