TC500 Microchip Technology, TC500 Datasheet - Page 13

• Manufacturer Part Number: TC500
• Description: (TC500 - TC514) Precision Analog Front Ends
• Manufacturer: Microchip Technology

8.0
8.1
The threshold noise (N
integrator and comparator noise and is typically 30 µV.
Figure 8-1 illustrates how the value of the reference
voltage can affect the final count. Such errors can be
reduced by increased integration times, in the same
way that 50/60 Hz noise is rejected. The signal-to-
noise ratio is related to the integration time (T
the integration time constant (R
EQUATION 8-1:
8.2
To obtain maximum performance from the TC5XX, the
overshoot at the end of the de-integration phase must
be minimized. Also, the integrator output zero phase
must be terminated as soon as the comparator output
returns high (see Figure 5-1).
Figure 5-1 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 having it control a timer directly for the
Reference de-integration phase (this is further
explained below.)
The timing diagram in Figure 5-1 is not to scale, as the
timing in a real system depends on many system
parameters and component value selections. There
are four critical timing events (as shown in Figure 5-1):
sampling the input polarity, capturing the de-integration
time, minimizing overshoot and properly executing the
integrator output zero phase.
8.3
The length of this phase is usually set to be equal to the
input signal integration time. This decision is virtually
arbitrary 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 indefinitely (i.e., auto-zero is the
appropriate Idle state for a TC5XX device).
 2004 Microchip Technology Inc.
S/N (dB)
DESIGN CONSIDERATIONS
Noise
System Timing
Auto-zero Phase
=
20 log



---------------------- -
30 10
TH
V
) is the algebraic sum of the
IN
6 –
INT
-------------------------------------- -
, C
R
INT
INT
t
) as follows:
INT
C
INT
INT



) and
8.4
The length of this phase is constant from one
conversion to the next and depends on system
parameters and component value selections. The
calculation of T
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 terminated
by changing AB from 10 to 11. The consideration here
is that, during the initial stage of input integration when
the integrator voltage is low, the comparator may be
affected by noise and its output unreliable. Once
integration is well underway, the comparator will be in a
defined state.
8.5
The length of this phase must be precisely measured
from the transition of AB from 10 to 11 to the falling-
edge of CMPTR. The comparator delay contributes
some error in timing this phase. The typical delay is
specified to be 2 µsec. This should be considered in the
context of the length of a single count when
determining overall system performance and possible
single count errors. Additionally, overshoot will result in
charge accumulating on the integrator once its output
crosses zero. This charge must be nulled during the
integrator output zero phase.
Input Signal Integrate Phase
Reference De-integration
TC500/A/510/514
INT
is shown elsewhere in this data
DS21428C-page 13