C
Chris Carlen
Guest
Hi:
I'm planning to design a DSP board based on the TMS320F2812. I want to
add a better A/D, so am in the process of designing a front-end.
The required specs are 14-16 bits, at least two simultaneously sampled
channels at a time, with 8 channels total. Preferably differential
inputs, up to 100kHz full-power bandwidth and at most 4us to convert two
channels. Some configurability of input voltage ranges: unipolar ranges
of 10V, 5V, 1V, and 0.2V; bipolar ranges of +/-10V, +/-5V, +/-1V, and
+/-0.2V. It's ok for the bandwidth to be reduced somewhat when using
gain greater than 1.
As for the A/D I'm thinking of utilizing the Maxim MAX1324 8-channel
simultaneous 250ksps 14-bit converter with +/-10V input range. This
chip also has variants with +/-5V and 0-5V inputs.
I'd like to keep component count as low as possible. Three SO8 sized
chips per channel is the limit. Yet I want to be able to calibrate each
channel to have no more than 1-2LSB of offset, and no more than 0.05%
gain error (8 LSB out of 14 bits). I don't need software
configurability or calibration. Jumpers and pots is Ok.
My stumbling block is that to handle the +/-10V input range, the
traditional instrument amps like LT1167 will slew rate limit above about
19kHz. The use of such an amp is desirable since it is a convenient
place to inject an offset correction, as well as switching the overall
gain ranges. My planned front end topology is as follows (a separate
input signal chain per channel):
A differential input stage such as the LT1167 instrument amp,
followed by a 4-pole Bessel filter (this might be just a socket,
allowing pluggable filter modules to be inserted), followed by a fast
settling buffer amp to drive the A/D. In order to switch between
bipolar and unipolar input voltage ranges, the A/D driver amp will be in
a non-inverting configuration. It can be switched between a unity-gain
buffer (for bipolar inputs) or a x2 amplifier with -10V offset (for
unipolar inputs) by simply jumpering a resistor from the - terminal to a
+10V reference (it will have an equal valued feedback resistor, of
course). The only thing missing in this plan is a good way to tweak
overall gain to calibrate to my desired spec. That could be done by
tweaking the gain in the filter stage, though, or by tweaking the LT1167
gain resistor. The latter method wouldn't work for the 10V ranges
though. I plan to inject an offset adjustment to the LT1167's reference
terminal.
The MAX1324 has some offset and gain adjustment facilities, by using an
external reference and a "MSV (mid scale voltage) input pin. But the
channel to channel gain and offset error matching specs are less than my
requirements, so that's why I'd like to be able to individually
calibrate each channel for offset and gain.
It seems the only way to get fast differential inputs would be to roll
my own instrument amp stage.
Is there a better way?
On obvious question would be whether differential inputs are really
needed. The inputs are likely to be fed by signal sources external to
the DSP board, and carried over cables. In these situations and where
DC accuracy and low noise is required, I have always thought it
necessary to have differential inputs.
Thanks for inputs (no pun intended).
--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
crcarle@sandia.gov -- NOTE: Remove "BOGUS" from email address to reply.
I'm planning to design a DSP board based on the TMS320F2812. I want to
add a better A/D, so am in the process of designing a front-end.
The required specs are 14-16 bits, at least two simultaneously sampled
channels at a time, with 8 channels total. Preferably differential
inputs, up to 100kHz full-power bandwidth and at most 4us to convert two
channels. Some configurability of input voltage ranges: unipolar ranges
of 10V, 5V, 1V, and 0.2V; bipolar ranges of +/-10V, +/-5V, +/-1V, and
+/-0.2V. It's ok for the bandwidth to be reduced somewhat when using
gain greater than 1.
As for the A/D I'm thinking of utilizing the Maxim MAX1324 8-channel
simultaneous 250ksps 14-bit converter with +/-10V input range. This
chip also has variants with +/-5V and 0-5V inputs.
I'd like to keep component count as low as possible. Three SO8 sized
chips per channel is the limit. Yet I want to be able to calibrate each
channel to have no more than 1-2LSB of offset, and no more than 0.05%
gain error (8 LSB out of 14 bits). I don't need software
configurability or calibration. Jumpers and pots is Ok.
My stumbling block is that to handle the +/-10V input range, the
traditional instrument amps like LT1167 will slew rate limit above about
19kHz. The use of such an amp is desirable since it is a convenient
place to inject an offset correction, as well as switching the overall
gain ranges. My planned front end topology is as follows (a separate
input signal chain per channel):
A differential input stage such as the LT1167 instrument amp,
followed by a 4-pole Bessel filter (this might be just a socket,
allowing pluggable filter modules to be inserted), followed by a fast
settling buffer amp to drive the A/D. In order to switch between
bipolar and unipolar input voltage ranges, the A/D driver amp will be in
a non-inverting configuration. It can be switched between a unity-gain
buffer (for bipolar inputs) or a x2 amplifier with -10V offset (for
unipolar inputs) by simply jumpering a resistor from the - terminal to a
+10V reference (it will have an equal valued feedback resistor, of
course). The only thing missing in this plan is a good way to tweak
overall gain to calibrate to my desired spec. That could be done by
tweaking the gain in the filter stage, though, or by tweaking the LT1167
gain resistor. The latter method wouldn't work for the 10V ranges
though. I plan to inject an offset adjustment to the LT1167's reference
terminal.
The MAX1324 has some offset and gain adjustment facilities, by using an
external reference and a "MSV (mid scale voltage) input pin. But the
channel to channel gain and offset error matching specs are less than my
requirements, so that's why I'd like to be able to individually
calibrate each channel for offset and gain.
It seems the only way to get fast differential inputs would be to roll
my own instrument amp stage.
Is there a better way?
On obvious question would be whether differential inputs are really
needed. The inputs are likely to be fed by signal sources external to
the DSP board, and carried over cables. In these situations and where
DC accuracy and low noise is required, I have always thought it
necessary to have differential inputs.
Thanks for inputs (no pun intended).
--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
crcarle@sandia.gov -- NOTE: Remove "BOGUS" from email address to reply.