Adc Circuit Description - Scion Instruments 436-GC Service Manual

436-GC/456-GC

ADC Circuit Description

System description.
The central ADC system consists of a 16-bit ADC, several voltage references, a programmable-gain
amplifier (PGA), a group of analog multiplexers, excitation circuitry for the RTD temperature probes, a
barometric absolute pressure sensor, and timing logic to control the system. The logic, which relieves
software of any precise timing requirements, is contained in FPGA X2.
ADC.
The ADC is a 16-bit (or 15-bit plus sign) delta-sigma converter. The excellent noise-rejecting characteristics
of this conversion technique allow consistent noise levels below ±1/2LSB with the simple supply filtering
networks of R53, C59, R81, and C57. The clock frequency at XIN is 320 kHz.
Conversions are started 160 times per second by a hardware-generated pulse on pin 2. A conversion takes
5 milliseconds, after which DRDY\ goes low, generating an interrupt and latching a new, buffered value of
BP/UP\ (bipolar/unipolar mode). Software then has over 6 milliseconds to read the converted value and
update the buffer holding the next desired value of BP/UP\, before the data from the next conversion will be
ready. The converted value is read serially from SDATA, with hardware sending a pulse to SCLK each time
SDATA is read, thus shifting out the next bit. When all of the bits have been read, DRDY\ goes high again.
The differential input range of the ADC in unipolar mode is equal to the voltage difference between VREF+
and VREF-, or 2.5V. Since the converter is supplied from +5.25V only, the analog input zero level must be
shifted up above ground if negative values are to be converted in bipolar mode. AIN- is connected to
+2.5V, so the input range at AIN+ in unipolar mode is +2.5V to +5.0V, and the range in bipolar mode is OV
to +5V. Resistors R51, R54, and R55 shift and attenuate the ±10V levels from the PGA output to cover the
0 to +5V input range of the ADC. R50, C60, and C63 smooth the input charge packets which are
transferred when the ADC samples its input.
Programmable-gain amplifier and references.
Gains of 1, 2, 4, 8, 16, 32, and 64 are provided by the programmable-gain amplifier. Corresponding full-
scale voltages for the ADC, measured at the PGA input, range from 10V down to 156mV (nominal).
Resolution depends upon the conversion mode, ranging from 160microV to 2.51.N in unipolar mode, or
320 microV to 5 microV in bipolar mode (nominal). The gain-setting resistors in network R4 have only 1%
tolerance, so the overall gain (including R51, R54, and R55) is set slightly low to ensure that the nominal
full-scale inputs can be converted without clipping. Similarly, the ADC input voltage with zero volts into the
PGA is set slightly above 2.5V, so that PGA inputs all the way down to ground can always be digitized,
regardless of component tolerance variations.
Capacitor C62 reduces high-frequency noise from the amplifier, while C50 and C54 suppress digitally-
induced noise. R43 limits the current pulse which is drawn from the input voltage source when the input
multiplexer switches, charging C50 to a new voltage. The time constants set by these capacitors are short
enough to settle to the required accuracy much more quickly than the lms which is available between
conversions. In addition, the input multiplexer is set to ground for 200 micro s after each conversion. This
elminates any residual crosstalk between channels which might occur if the amplifier is driven into
saturation. Note: the muiltiplexer is set to ground by hardware, so this action cannot be stopped by holding
the multiplexer setting at a fixed channel in software. Voltmeter readings at the output of the muxtiplexer
will be about 3% lower than at the input in this case.
There are three precision reference voltages: +10V, +5V, and +2.5V. The basic accuracy of the +10V
reference (U9) is ±0.1%. C13 minimizes the glitches on the reference induced by the DAC switches, which
operate asynchronously to the ADC conversions. The other two values are divided down from this
reference by resistors in network R4 which have ±0.1% ratio accuracy, giving guaranteed accuracies of
±0.2% for the +5V and +2.5V references. One half of AR5 buffers the +2.5V reference for the ADC.
The other half of AR5, along with Ql, can supply up to 40mA of excitation current at +5V to the RTDs, the
barometric absolute pressure transducer, and the EFC modules.
SCION Instruments
436-GC/456-GC Service Manual Revision B February 2019
610 Hardware description
Page: 79