PRECISION ANALOG FRONT ENDS
TC500
TC500A
TC510
TC514
Table 1. CREF and CAZ Selection
APPLICATIONS
Conversions Typical Value of Suggested *
Component Value Selection
Per Second
CREF, CAZ (µF) Part Number
The procedure outlined below allows the user to arrive
at values for the following TC5xx design variables:
>7
2 to 7
2 or less
0.1
0.22
0.47
WIMA MK12 .1/63/20
WIMA MK12 .22/63/20
WIMA MK12 .47/63/20
(1) Integration Phase Timing
*WIMA Corp. listing on the last page of this data sheet.
(2) Integrator Timing Components (RINT, CINT
(3) Auto Zero and Reference Capacitors
(4) Voltage Reference
)
Calculate Integrating Capacitor (CINT
The integrating capacitor must be selected to maximize
integrator output voltage swing. The integrator output volt-
age swing is defined as the absolute value of VDD
less0.9V(i.e., VDD –0.9V or
)
Select Integration Time
(or
)
VSS
VSS +0.9V ). Usingthe20µA
Integration time must be picked as a multiple of the
period of the line frequency. For example, tINT times of
33msec, 66msec and 132msec maximize 60Hz line rejec-
tion.
buffer maximum output current, the value of the integrating
capacitor is calculated using the following equation.
(tINT) (20 x 10 –6
(VS – 0.9)
)
CINT = (in µF) =
DINT and IZ Phase Timing
The duration of the DINT phase is a function of the
amount of voltage stored on the integrator during TINT, and
the value of VREF. The DINT phase must be initiated imme-
diately following INT and terminated when an integrator
output zero-crossing is detected. In general, the maximum
number of counts chosen for DINT is twice that of INT (with
VREF chosen at VIN (max)/2).
where: tINT = Integration Period
VS = Applied Supply Voltage
It is critical that the integrating capacitor has a very low
dielectric absorption. Polypropylene capacitors are an ex-
ampleofonesuchchemistry.PolyesterandPolybicarbonate
capacitors may also be used in less critical applications.
Table 2 summarizes recommended capacitors for CINT
.
Calculate Integrating Resistor (RINT
)
The desired full-scale input voltage and amplifier output
current capability determine the value of RINT. The buffer
and integrator amplifiers each have a full-scale current of
20µA.
Table 2. Recommend Capacitor for CINT
Value
Suggested Part Number*
0.1
WIMA MK12 .1/63/20
WIMA MK12 .22/63/20
WIMA MK12 .33/63/20
WIMA MK12 .47/63/20
0.22
0.33
0.47
The value of RINT is therefore directly calculated as
follows:
VIN MAX
RINT(in MΩ) =
20
*WIMA Corp. listing on the last page of this data sheet.
where: VIN MAX = Maximum input voltage (full count voltage)
RINT =Integrating Resistor (in MΩ)
For loop stability, RINT should be ≥ 50kΩ.
Calculate VREF
The reference deintegration voltage is calculated
using:
Select Reference (CREF) and Auto Zero (CAZ) Capacitors
CREF and CAZ must be low leakage capacitors (such as
polypropylene). The slower the conversion rate, the larger
the value CREF must be. Recommended capacitors for CREF
and CAZ are shown in Table 1. Larger values for CAZ and
CREF may also be used to limit roll-over errors.
(VS – 0.9) (CINT) (RINT
)
VREF (in Volts) =
2(tINT
)
3-28
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