TC500 Datasheet, PDF (10/17 Pages) TelCom Semiconductor, Inc – PRECISION ANALOG FRONT ENDS

TC500
TC500A
TC510
TC514
PRECISION ANALOG FRONT ENDS
APPLICATIONS
Component Value Selection
The procedure outlined below allows the user to arrive
at values for the following TC5xx design variables:
(1) Integration Phase Timing
(2) Integrator Timing Components (R_INT, C_INT)
(3) Auto Zero and Reference Capacitors
(4) Voltage Reference
Select Integration Time
Integration time must be picked as a multiple of the
period of the line frequency. For example, t_INT times of
33msec, 66msec and 132msec maximize 60Hz line rejec-
tion.
D_INT and I_Z 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 V_REF. The DINT phase must be initiated imme-
diately following I_NT 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
V_REF chosen at V_IN (max)/2).
Calculate Integrating Resistor (R_INT)
The desired full-scale input voltage and amplifier output
current capability determine the value of R_INT. The buffer
and integrator amplifiers each have a full-scale current of
20µA.
The value of R_INT is therefore directly calculated as
follows:
R_INT(in
MΩ)
=
V_{IN MAX}
20
where: V_{IN MAX} = Maximum input voltage (full count voltage)
R_INT =Integrating Resistor (in MΩ)
For loop stability, R_INT should be ≥ 50kΩ.
Select Reference (C_REF) and Auto Zero (C_AZ) Capacitors
C_REF and C_AZ must be low leakage capacitors (such as
polypropylene). The slower the conversion rate, the larger
the value C_REF must be. Recommended capacitors for C_REF
and C_AZ are shown in Table 1. Larger values for C_AZ and
C_REF may also be used to limit roll-over errors.
Table 1. C_REF and C_AZ Selection
Conversions Typical Value of Suggested *
Per Second C_REF, C_AZ (µF) Part Number
>7
0.1
WIMA MK12 .1/63/20
2 to 7
0.22
WIMA MK12 .22/63/20
2 or less
0.47
WIMA MK12 .47/63/20
*WIMA Corp. listing on the last page of this data sheet.
Calculate Integrating Capacitor (C_INT)
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 V_DD (or V_SS)
less 0.9V (i.e., V_DD – 0.9V or V_SS +0.9V ). Using the 20µA
buffer maximum output current, the value of the integrating
capacitor is calculated using the following equation.
C_INT = (in µF) = ^{(tINT) (20 x 10 –6)}
(V_S – 0.9)
where: t_INT = Integration Period
V_S = Applied Supply Voltage
It is critical that the integrating capacitor has a very low
dielectric absorption. Polypropylene capacitors are an ex-
ample of one such chemistry. Polyester and Polybicarbonate
capacitors may also be used in less critical applications.
Table 2 summarizes recommended capacitors for C_INT.
Table 2. Recommend Capacitor for C_INT
Value
Suggested Part Number*
0.1
WIMA MK12 .1/63/20
0.22
WIMA MK12 .22/63/20
0.33
WIMA MK12 .33/63/20
0.47
WIMA MK12 .47/63/20
*WIMA Corp. listing on the last page of this data sheet.
Calculate V_REF
The reference deintegration voltage is calculated
using:
V_REF
(in
Volts)
=
(V_S
–
0.9) (C_INT)
2(t_INT)
(R_INT)
3-28
TELCOM SEMICONDUCTOR, INC.