VTI Instruments EX1401, User Manual

Page: 100 / 114

VTI Instruments Corp.
100 EX1401 Theory of Operation
normal operating range of the PGA and do not disturb the accuracy of the measurement (for normal
mode measurements.) When a channel is in overvoltage mode, it can (optionally) be detected by the
software and that channel’s LED will light.
Each input has open transducer detection. This is particularly useful if a thermocouple connection
opens up. This condition can be (optionally) detected by the software and that channel’s LED will
light. The open transducer detection presents a very small current (typically less than 10nA) into
both sides of the mini TC connector inputs. The current will normally flow through the
thermocouple wire from the (+) side to the (-) side. As long as the thermocouple wire presents a
short circuit (as it normally does) then there is no fault detected. These current sources are small
enough not to affect measurement accuracy, but large enough to drive the high-impedance input of
the instrumentation amplifier (IA) deterministically into saturation in the event of an open condition.
When enabled, they provide continuous monitoring of the input and will generate an open indication
event when the open is intermittent in nature. An important feature of the OTD is that the current
sources can be enabled or disabled on a per channel basis. This permits the user to choose which of
their input signals require open circuit detection.
P
ROGRAMMABLE
G
AIN
A
MPLIFIER
Each channel has its own dedicated analog signal conditioning chain as well as a dedicated ADC,
reference, and BIST circuit.
There are two low pass filters in the chain, one prior to the programmable gain amplifier (PGA) and
one prior to the ADC. Both are single pole filters with a nominal 3dB cutoff at 30kHz. These two
filters, along with a filter in the PGA section, provide the Nyquist frequency rejection for the ADC.
These analog filters keep anti-aliasing of high frequency to a minimum so the digital data output of
the ADC can then be confidently filtered by the subsequent digital filters inside the FPGA.
The PGA amplifies the input differential signal and outputs a differential signal to the subsequent
LPF. The ADC then converts the differential signal to digital sampled data. The PGA provides
excellent common mode rejection (typically -130dB at 50/60Hz and -120dB at 1kHz).
A
NALOG TO
D
IGITAL
C
ONVERSION
Each channel has its own, dedicated ADC that samples the analog signal at some rate between
320kSPS and 640kSPS (depending upon customer selected sample rate.) The ADC is a successive
approximation with register (SAR) type converter. The ADC oversamples the analog input signal
by a factor of 32 and then performs digital filtering (linear phase response) to eliminate out of band
noise. The ADC generates 24-bit samples at the reduced output data rate of 10kSPS to 20kSPS.
Therefore, the effective analog sample rate is somewhere between 10kSPS and 20kSPS, depending
upon customer selected sample rate. The ADC has excellent AC and DC characteristics that allow
the EX1401 to perform more than just thermocouple measurements. The AC response, along with
the analog and digital filtering allow applications beyond thermocouples.
All channels on the EX1401 are totally independent, but are synchronized by the same clock.
Therefore, all ADCs in the system sample their analog inputs at the same time and generate output
data at the same rate.
B
UILT
I
N
S
ELF
-
TEST AND
CJC
MEASUREMENT
Each channel has its own, dedicated built-in-self-test circuit which includes a separate ADC from
the measurement path. The BIST ADC operates at a slower rate, 50 to 100 Samples/Sec depending
upon customer selected sample rate. This lower sample rate provides the high accuracy needed for
CJC measurement and reference measurement.
The BIST ADC is responsible for measuring the output of the thermistor used for CJC. The
thermistor is part of a resistor divider circuit driven by the on board reference. The BIST ADC can
measure both the reference and the thermistor output so only a ratio metric calculation needs to be