Sensors & transducers
first-choice circuit element chosen to drive the finicky input of an ADC (some ADCs are more finicky than others). There is so much that an in- amp already does that it would seem unfair to desire that it even do more! Overcoming the harmonic distortion (HD) of
an ADC driver is a difficult challenge. Here is the expression for what kind of distortion an ADC driver would have to meet or exceed as a function of the ADC resolution:
SINAD: SNR + distortion ENOB: Effective number of bits So, for an ENOB of 16 bits, SINAD ≥ 98 dB
The in-amps currently available on the market
are usually not meant to drive an ADC input. The most common reason for this is that these devices lack the linearity that a high resolution ADC necessitates. Linearity, or harmonic distortion (also called THD, the total harmonic distortion), is the most likely limiting factor that would prevent an in- amp from being able to drive an ADC directly. When a complex waveform is digitised, once contaminated with distortion terms, the signal becomes indistinguishable from any such contamination and thus the data acquisition is compromised! The driver should also be able to settle quickly from the ADC input charge injection transient explained earlier.
Current SolutionS improved
With the new instrumentation amplifier family, we now have a device family that does all that an in-amp has traditionally done plus it can now drive an ADC directly very well and protect the ADC input as well. The LT6372-1 (for gains from 0 dB to 60 dB) and the LT6372-0.2 (for gains/attenuation from –14 dB to +46 dB) can help fulfill the task of a precision transducer interface that can directly drive an ADC input. There are obvious advantages to using a high
precision, low noise instrumentation amplifier such as the LT6372 family to drive an ADC analog input directly without the need to add another amplification or buffering stage. Some of these benefits are reduced component count, power consumption, cost, and board area, as well as high CMR, excellent dc precision, low 1/f noise, and single component gain selection.
Figure 4. LT6372-1 and LT6372-0.2 frequency response at various gains. Many high speed op amps selected as ADC
drivers may not have the low 1/f noise that the LT6372 family has due to the proprietary process it is built on. Furthermore, additional buffering and gain stages may have to be added to amplify the small transducer signal. With direct in-amp ADC driving, there are no additional noise sources or dc offset terms from amplifier stages or voltage references to contend with either. The LT6372-1 and LT6372-0.2, with their
extremely high input impedance, can interface a transducer or similar signal input and provide large gain (LT6372-1) or attenuation (LT6372- 0.2) without causing loading, while their low distortion and low noise assure accurate conversion without degradation for 16-bit and lower resolution ADCs at up to 150 kSPS. Figure 4 shows the bandwidth that each device can achieve for a given gain setting. Refer to Figure 5 for the LT6372-1 distortion
vs. frequency to make sure the distortion terms do not significantly dominate the THD performance of the ADC being considered at the highest frequency of interest. For an example of an ADC, LTC2367-16 has a SINAD specification of 94.7 dB. To make sure the driver is not dominant, Figure 5 shows that LT6372-1 would be a suitable choice for frequencies less than ~5 kHz.
the nitty Gritty of uSinG lt6372-1 aS an adC driver
In addition to the advantages previously noted, the split-reference architecture of the LT6372 family (shown in Figure 6 as separate RF1 and RF2 pins) allows an elegant way to shift the
Figure 5. LT6372-1 THD vs. frequency.
signal to within the ADC FS voltage range directly and efficiently, without having to use additional voltage references and other external circuitry to achieve the same, thereby reducing cost and complexity. For most ADCs, REF2 (shown tied to the VOCM dc voltage
here) would be tied to the ADC VREF voltage, which would ensure the ADC analog input mid-level to be VREF/2.
Figure 6. LT6372 split-reference used to shift signal to an ADC analogue input.
The LT6372 family built-in output clamps
(CLHI and CLLO) ensure that the sensitive input of the ADC is not violated or possibly harmed from transients in either the positive or negative direction. They allow undistorted output swing right up to the clamp voltage with fast response and recovery to provide ADC protection and quick return to normal operation after a possible transient that could trigger either clamp. The analogue input of some SAR ADCs
Figure 3. An ideal transducer amplifier/ADC driver visualised.
presents a challenging load for an amplifier to drive. The amplifier needs to be low noise and fast settling, as well as possess high dc precision to keep unwanted signal perturbations to one LSB or less. Higher sampling rates and higher order ADCs place more demand on the amplifier. Figure 7 shows the input of a typical SAR ADC.
Instrumentation Monthly February 2021
Continued on page 36... 35
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