Download Topic: High Performance Data Acquisition Systems Analog

Topic: High Performance Data Acquisition Systems
Analog Components: Differentially Driving Analog to Digital Convertors
Figure 1 The Most Simple High Performance Data Acquisition System
Let’s conclude our series on high performance data acquisition systems. We have discussed
many different key aspects of various types of data acquisition systems and how to design and
develop them to achieve the overall desired result. The most elementary system architecture
configuration would be to simply connect the sensor output directly to the analog to digital
convertor input and therefore bypass any other signal conditioning amplifiers leading up the
ADC (see Figure 1). Unfortunately, for most high performance systems, this simple
configuration is not possible due to the complex input drive requirements necessary for today’s
high performance ADC’s. In fact, as previously discussed, many high frequency ADC’s have unbuffered differential inputs that require extreme caution when designing and selecting a
suitable differential drive amplifier. Remember, un-buffered ADC “inputs” often times become
“outputs” and transmit signals to the outputs of the ADC input buffer amplifier(s) that become
superimposed on the input voltage (of the ADC) and the ADC will then convert a corrupted
input signal that is ultimately caused by the ADC itself! Last week we looked at passive
differential ADC input drivers, this week let’s finish up with active ADC drivers.
Figure 2 Single-ended to Single-ended Active ADC Input Driver
Figure 2 shows a simple single-ended to single-ended active ADC input driver (using a CADEKA
CLC1003 low distortion (HD2=-125 dBc and HD3=-127 dBc) and low noise (en=3.5 nV/√Hz)
amplifer). In a single-ended to single-ended application, a high frequency, low distortion
amplifier (like the CLC1003) is used in this application to drive an ADC with a single-ended input
or one side of a differential input (with the other differential input connected to an ADC middle
input common mode voltage reference). This type of inverting amplifier configuration is useful
in many cases because a reference voltage can be applied to the (+) terminal of the amplifier to
ultimately reference the mid-point of the input signal. Capacitors such as a .01uf and .1uF at the
(+) terminal of the amplifier can also be used to filter any unwanted noise that may be induced
into the circuit through the reference voltage source and divider. An RC filter on the output of
the amplifier can also band-limit some of the amplifiers output noise, but be careful, that
resistor can also act as a multiplier of an un-buffered ADC output glitch and cause an input
voltage glitch if the resistance is too high. Of course the gain of the amplifier is set by G=-R2/R1.
Figure 3 Single-ended to Differential Active ADC Input Driver
A single-ended to differential ADC driver is shown in Figure 3, and it utilizes a dual op amp to
buffer a single-ended source to drive an ADC with differential inputs. One of the op amps is
configured as a unity gain buffer that drives the inverting input of the other op amp while also
driving the non-inverting input of the ADC. The ADC driver is configured for a gain of 2 in order
to reduce the noise without sacrificing the harmonic distortion performance. A common mode
voltage of 2.5V is usually supplied at the non-inverting inputs of both op amps.
This configuration produces a differential +/- 2.5 Vpp output signals when the single-ended
input signal of 0 to Vref is AC coupled into the non-inverting terminal of the op amp and each
non-inverting terminal of the op amp is biased at a mid-scale voltage of +2.5V. Two output RC
anti-aliasing filters are used between the outputs of both op amps and the differential inputs of
the ADC in order to minimize the effect of undesired high frequency noise coming from the
input source. Each RC filter’s cut-off frequency is found from the equation fcut-off=1/2πRC.
Figure 4 Differential to Differential Active ADC Input Driver
A differential to differential active ADC input driver is shown in Figure 4. Just as a dual op amp
can be configured to perform a single-ended to differential conversion, so to a dual op amp can
be configured as a differential to differential ADC driver to buffer a differential source to
differential ADC inputs. A differential to differential ADC driver can be formed by using two
single-ended to single-ended ADC drivers. Each output from these drivers can go into separate
inputs of a differential ADC. In this example, each single to single ADC driver uses the same
components and is in a multi-feedback inverting configuration.
One thing that is important to note when driving ANY high performance ADC is to pay special
attention to the signal ground reference as well as the power supply ground reference. It is
important to connect the input source ground with the power supply ground. Any voltage
potential difference between the two will usually result in an interference/noise and distortion
signal picked up by the ADC. Make sure the power supply grounds (for the ADC) and the signal
grounds are connected together at the ADC on the PCB. Also, for each ADC driver configuration,
it is also important to account for the impedances of the signal sources when setting up the
gain setting resistor networks in order to ensure that the differential outputs have the same
gain. Any signal source impedance must be added to the gain setting resistors to ensure proper
gain adjustments.
Again, when designing a high performance (and high frequency) data acquisition system,
because the analog to digital convertor is usually the limiting factor in overall system resolution,
accuracy, and noise, great care must be taken in driving it’s inputs. Whether the convertor has
single-ended inputs (which require an ultra stable external driver amplifier with very stringent
DC specifications) or differential inputs (which require an ultra stable on-board ADC voltage
reference and external amplifier with differential outputs), the designer needs to know the
system level trade-offs for each configuration. As always, choosing the proper ADC, and
correlating its DC specifications to the key results that you desire the system to perform, will
then set the limit of most of your system level performance parameters.
Kai ge from CADEKA