34
LTC2404/LTC2408
APPLICATIONS INFORMATION
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amplifier-based or self-contained instrumentation ampli-
fiers (also available from LTC) can be used with the
LTC2408.
With the resistor network connected to CH0, the LTC2408
is able to measure DC voltages from 1mV to 1kV in a single
range without the need for autoranging. The 990k resistor
should be a 1W resistor rated for high voltage operation.
Alternatively, the 990k resistor can be replaced with a
series connection of several lower cost, lower power metal
film resistors.
The circuit connected to CH1 shows an LT1793 FET input
operational amplifier used as an electrometer for high
impedance, low frequency applications such as measur-
ing pH. The circuit has been configured for a gain of 21;
thus, the input signal range is –15mV ≤ V
IN
≤ 250mV. An
amplifier circuit is necessary in these applications be-
cause high output impedance sensors cannot drive
switched-capacitor ADCs directly. The LT1793 was cho-
sen for its low input bias current (10pA, max) and low
noise (8nV/√Hz) performance. As shown, the use of a
driven guard (and Teflon
TM
standoffs) is recommended in
high impedance sensor applications; otherwise, PC board
surface leakage current effects can degrade results.
The circuit connected to CH2 illustrates a precision half-
wave rectifier that uses the LTC2408’s internal ∆Σ ADC as
an integrator. This circuit can be used to measure 60Hz,
120Hz or from 400Hz to 1kHz with good results. The
LTC2408’s internal sinc
4
filter effectively eliminates any
frequency in this range. Above 1kHz, limited amplifier
gain-bandwidth product and transient overshoot behavior
can combine to degrade performance. The circuit’s dy-
namic range is limited by operational amplifier input offset
voltage and the system’s overall noise floor. Using an
LTC1050 chopper-stabilized operational amplifier with a
V
OS
of 5µV, the dynamic range of this application covers
approximately 5 orders of magnitude. The circuit configu-
ration is best implemented with a precision, 3-terminal,
2-resistor 10k network (for example, an IRC PFC-D net-
work) for R6 and R7 to maintain gain and temperature
stability. Alternatively, discrete resistors with 0.1% initial
tolerance and 5ppm/°C temperature coefficient would
also be adequate for most applications.
Two channels (CH3 and CH4) of the LTC2408 are used to
accommodate a 3-wire 100Ω, Pt RTD in a unique circuit
that allows true RMS/RF signal power measurement from
audio to gigahertz (GHz) frequencies. The unique feature
of this circuit is that the signal power dissipated in the 50Ω
termination in the form of heat is measured by the 100Ω
RTD. Two readings are required to compensate for the
RTD’s lead-wire resistance. The reading on CH4 is multi-
plied by 2 and subtracted from the reading on CH3 to
determine the exact value of the RTD.
While the LTC2408 is capable of measuring signals over a
range of six decades, the implementation (mechanical,
electrical and thermal) of this technique ultimately deter-
mines the performance of the circuit. The thermal resis-
tance of the assembly (the 50Ω/RTD mass to its enclosure)
will determine the sensitivity of the circuit. The dynamic
range of the circuit will be determined by the maximum
temperature the assembly is rated to withstand, approxi-
mately 850°C. Details of the implementation are quite
involved and are beyond the scope of this document.
Please contact LTC directly for a more comprehensive
treatment of this implementation.
In the circuit connected to the LTC2408’s CH5 input, a
thermistor is configured in a half-bridge arrangement that
could be used to measure the case temperature of the
RTD-based thermal power measurement scheme described
previously. In general, thermistors yield very good resolu-
tion over a limited temperature range. Measurement reso-
lution of 0.001°C is possible; however, thermistor
self-heating effects, thermistor initial tolerance and circuit
thermal construction can combine to limit achievable
resolution. For the half-bridge arrangement shown, the
LTC2408 can measure temperature changes over 5 orders
of magnitude.
Connected to the LTC2408’s CH6 input, an infrared ther-
mocouple (Omega Engineering OS36-1) can be used in
limited range, noncontact temperature measurement ap-
plications or applications where high levels of infrared
light must be measured. Given the LTC2408’s 0.3ppm
RMS
noise performance, measurement resolution using infra-
red thermocouples is approximately 0.03°C—equivalent
to the resolution of a conventional Type J thermocouple.
Teflon is a trademark of Dupont Company.
