參數(shù)資料
型號： TC500
廠商： Microchip Technology Inc.
英文描述： Precision Analog Front Ends
中文描述： 精密模擬前端
文件頁數(shù)： 11/17頁
文件大小： 209K
代理商： TC500
3-29
TELCOM SEMICONDUCTOR, INC.
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8
PRECISION ANALOG FRONT ENDS
TC500
TC500A
TC510
TC514
Figure 6. Noise
S
N
TH
Low
REF
S
N
TH
Normal
V
REF
S
N
TH
High
30
μ
V
V
REF
SLOPE (S) =
N
TH
= Noise Threshold
R
INT
C
INT
V
REF
Figure 7. Overshoot
INTEGRATOR
OUTPUT
COMPARATOR
OUTPUT COMP
INTEGRATE
PHASE
DEINTEGRATE PHASE
INTEGRATOR
ZERO PHASE
ZERO
CROSSING
OVERSHOOT
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
having it control a timer directly for the Reference Deintegra-
tion phase. (This is further explained below.)
The timing diagram in Figure 8 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 8): sampling the input
polarity; capturing the deintegration time; minimizing over-
shoot and properly executing the Integrator Output Zero
phase.
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-Zero is the appropriate idle state for a TC5xx
device.
Input Signal Integrate Phase
The length of this phase is constant from one conversion
to the next and depends on system parameters and compo-
nent value selections. The calculation of t
INT
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
)
DESIGN CONSIDERATIONS
Noise
The threshold noise (NTH) is the algebraic sum of the
integrator noise and the comparator noise. This value is
typically 30
μ
V. 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 (t
INT
) and the integration time
constant (R
INT)
(C
INT
) as follows:
V
IN
30 x 10
–6
System Timing
To obtain maximum performance from the TC5xx, the
overshoot at the end of the Deintegration phase must be
minimized. Also, the Auto Zero phase must be terminated as
soon as the comparator output returns high. (See timing
diagram, Figure 8).
S/N (dB) = 20 Log
t
INT
(R
INT
) (C
INT
)
(
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