Is there a module that will allow me to directly convert the tiny sensor output signal to an ADC input voltage?

The latest family of instrumentation amplifiers from Analog Devices can accomplish the following tasks in one fell swoop: reject common-mode signals, amplify differential-mode signals, convert the voltage to meet the required ADC input voltage, and protect the ADC from overvoltage!

A common challenge in countless industrial, automotive, instrumentation, and many other applications is how to properly connect tiny sensor signals to an ADC for digitization and data acquisition. Sensor signals are often small, can be very noisy, and appear to be a very high impedance source sitting on top of a large common-mode (CM) voltage. None of this is what the ADC inputs like. This article will introduce a new integrated solution that can completely solve the problem that engineers have raised that is beyond the scope of current capabilities. It will also detail the design steps to configure a complete sensor interface instrumentation amplifier to drive the ADC input.

Figure 1. Challenges in getting from the sensor to the ADC

The short answer to this question is an instrumentation amplifier. The ideal object for a sensor to connect to is an instrumentation amplifier.

Instrumentation amplifiers have high precision (low offset) and low noise characteristics, and will not corrupt small input signals. Their differential inputs are suitable for many sensor signals (such as strain gauges, pressure sensors, etc.), and they are able to reject any common-mode signals that are present, leaving only the original small voltage of interest without the unwanted common-mode signal. Instrumentation amplifiers have large input impedance and do not load the sensor, ensuring that fragile signals are not affected by signal processing. In addition, instrumentation amplifiers can usually provide large gains and selectable gain ranges using a single external resistor, making them very flexible, allowing the target small signal to adapt to voltages and ADC analog inputs that are far above the noise level of the signal path. Instrumentation amplifiers are designed for precision performance and are internally trimmed to maintain their performance over a wide operating temperature range and are not affected by changes in the power supply voltage. Instrumentation amplifiers also have very low gain errors, which also helps them maintain accuracy and limit measurement or signal errors caused by swing changes.

_{Driving ADC inputs is not that easy. The switching action of the internal capacitors (C DAC} in Figure 2 ) of the front end causes charge injection, which makes it a difficult task to transmit a stable signal with high linearity for quantization by the ADC. The driver driving the ADC input must be able to handle these large charge injections and quickly settle before the next conversion cycle. In addition, depending on the ADC resolution (number of bits), the noise and distortion of the driver should not be a limiting factor.

Figure 2. ADC input driving is challenging

Meeting the above requirements is no small feat, especially for low-power drivers. In addition, due to modernization of semiconductor processes, ADC operating supply voltages are decreasing. One of the undesirable side effects of this trend is that the ADC inputs become more susceptible to input overvoltages, which can cause harm or damage. This requires external circuitry to protect against such overvoltages. Such external circuitry must not add any measurable noise to the signal, nor limit the bandwidth or introduce any form of distortion. It is also highly desirable that the entire circuit react quickly and recover quickly from overvoltage events.

There are also challenges in offsetting the input signal to match the ADC analog input voltage range. Any circuit elements added to perform this task must adhere to all of the constraints listed previously (i.e., low distortion, low noise, sufficient bandwidth, etc.).

All instrumentation amplifiers on the market have some drawbacks that require more circuit elements to complete the path from the physical world (sensor) to the digital world (ADC). Traditionally, instrumentation amplifiers have not been the first choice of circuit element to drive ADCs (some ADCs are more sophisticated than others). Instrumentation amplifiers do enough already, and it seems unfair to expect them to do more!

Overcoming harmonic distortion (HD) in ADC drivers is a difficult challenge. Below is an expression for the distortion performance that the ADC driver must meet or exceed as a function of the ADC resolution:

SINAD: SNR + Distortion

ENOB: Effective Number of Bits

Therefore, for 16-bit ENOB, SINAD ≥ 98 dB

Instrumentation amplifiers on the market today are generally not designed to drive ADC inputs. The most common reason for this is that these devices lack the linearity required for high-resolution ADCs. Linearity or harmonic distortion (also known as THD, or total harmonic distortion) are most likely limiting factors, preventing the instrumentation amplifier from driving the ADC directly. Once a complex waveform is digitized and corrupted by distortion terms, the signal cannot be distinguished from such interference and the data acquisition is corrupted! The driver should also be able to quickly settle from the ADC input charge injection transients explained previously.

Now, a new family of instrumentation amplifiers not only does everything that instrumentation amplifiers traditionally do, but also drives ADCs directly and protects ADC inputs extremely well! The LT6372-1 (supports gains from 0 dB to 60 dB) and LT6372-0.2 (supports gain/attenuation from –14 dB to +46 dB) can help with the task of precision sensor interfacing, driving ADC inputs directly.

Figure 3. Ideal sensor amplifier/ADC driver

There are clear advantages to using a high precision, low noise instrumentation amplifier such as the LT6372 family to directly drive the ADC analog inputs, eliminating the need for an additional amplification or buffer stage. Some of these benefits include: reduced component count, lower power and cost, smaller board area, high CMR, excellent dc precision, low 1/f noise, and gain selectability through a single component.

Many high speed op amps selected as ADC drivers may not have the low 1/f noise characteristics of the LT6372 family due to the proprietary process used to manufacture it. In addition, additional buffering and gain stages may need to be added to amplify the small sensor signals. When using an instrumentation amplifier to directly drive the ADC, there are no additional noise sources or dc offset terms comparable to those in the amplifier stage or voltage reference.

The LT6372-1 and LT6372-0.2 have extremely high input impedance to interface with sensors or similar signal inputs and provide large gain (LT6372-1) or attenuation (LT6372-0.2) without causing loading effects, while their low distortion and low noise ensure accurate conversion without degradation, supporting 16-bit and lower resolution ADCs running at rates up to 150 kSPS. Figure 4 shows the bandwidth that can be achieved with each device at a given gain setting.

Figure 4. Frequency response of the LT6372-1 and LT6372-0.2 at various gains

The relationship between LT6372-1 distortion and frequency is shown in Figure 5. It should be ensured that the distortion terms do not affect the THD performance of the ADC at the highest target frequency. For example, the ADC LTC2367-16 has a SINAD specification of 94.7 dB. To ensure that the driver is not the dominant factor, Figure 5 shows that the LT6372-1 is a suitable choice for frequencies less than about 5kHz.

Figure 5. LT6372-1 THD vs. frequency

In addition to the previously mentioned advantages, the split reference architecture of the LT6372 family (shown as separate RF1 and RF2 pins in Figure 6) allows the signal to be effectively shifted directly to the ADC FS voltage range without the need for an additional reference and other external circuitry to achieve the same end, thereby reducing cost and complexity. For most ADCs, REF2 (shown here tied to the V _OCM dc voltage) will be tied to the ADC V _REF voltage, which will ensure that the ADC analog input midscale is V _REF /2.

Figure 6. The LT6372 split reference voltage is used to shift the signal into the ADC analog input signal range.

The LT6372 family’s built-in output clamps (CLHI and CLLO) ensure that the sensitive inputs of the ADC are not damaged or potentially destroyed by positive or negative-going transients. The family allows for distortion-free output swings up to the clamp voltages and responds and recovers quickly, protecting the ADC and enabling it to quickly resume normal operation after a possible transient triggers either clamp.

The analog inputs of some SAR ADCs present challenging loads for the amplifier to drive. The amplifier needs to have low noise and fast settling characteristics, and high DC precision to keep the perturbation of the interfering signal to one LSB or less. Higher sampling rates and higher-order ADCs also place higher demands on the amplifier. Figure 7 shows the input of a typical SAR ADC.

Figure 7. SAR ADC input in acquisition/sampling mode

The switch position shown in Figure 7 corresponds to the sampling or acquisition mode, in which the analog input is connected to the sampling capacitor C _DAC before the conversion begins in the next operating phase.

Before this phase begins, switch S2 has discharged the C _DAC voltage to 0 V or another bias point, such as FS/2. At the beginning of the sampling period, S1 is closed and S2 is open, and the voltage difference between VSH and the analog input causes a transient current to flow, allowing C _DAC to charge to the analog input voltage. For higher sampling rate ADCs, this current can be as high as 50 mA. Capacitor C _EXT helps to mitigate the step change in the amplifier output voltage caused by this current step, but the amplifier will still be disturbed by it and need to settle in time before the end of the acquisition period. Resistor R _EXT isolates the driver from C _EXT and also reduces its impact on stability when driving large capacitors. The choice of R _EXT and C _EXT values ​​requires a trade-off between the greater isolation caused by this current injection and the degraded settling time performance caused by the low-pass filter formed in this way. This filter also helps to reduce out-of-band noise and improve SNR, but this is not its primary function.

There are many factors to consider when choosing the values ​​of R _EXT and C _EXT . The following is a summary of factors that affect the dynamic response of the ADC measured by FFT or other means:

Here are some design steps for designing R _EXT and C _EXT values, using the example of an LT2367-16 ADC driven by an LT6372-1 with a maximum input frequency f _IN of 2kHz and a sampling rate of 150 kSPS (see Reference 1 for a complete derivation of some of the following equations):

Choose a large enough CEXT _to act as a charge bucket and minimize charge kickback:

in:

C _DAC : ADC input capacitance = 45 pF (LTC2367-16)

→ C _EXT = 10 nF (selected value)

Use the following formula to calculate the ADC input voltage step, V _STEP :

in:

V _REF = 5 V (LTC2367-16)

C _DAC : ADC input capacitance = 45 pF (LTC2367-16)

C _EXT = 10 nF (before)

→ V _STEP = 22 mV (calculated value)

NOTE: This V _STEP function assumes that the C _DAC is discharged to ground at the end of each sampling period, as is the case for the LTC2367-16.

Assuming the step input settles exponentially, calculate how many input R _EXT ×C _EXT time constants N _TC are required to settle:

in:

V _STEP : ADC input voltage step calculated previously

V _{HALF_LSB} : LSB/2 in volts. For 5 V FS and 16 bits, this is 38 µV (= 5 V/2 ¹⁷ )

→ N _TC = 6.4 time constants

Calculate the time constant τ:

in:

t _ACQ : ADC acquisition time; t _ACQ = t _CYC – t _HOLD

Assuming a sampling rate of 150 kSPS:

tCYC ₌ 6.67 μs (= 1/150 kHz)

tHOLD ₌ 0.54 μs (LTC2367-16)

Therefore: t _ACQ = 6.13 μs

→ τ ≤ 0.96 µs

When τ and C _EXT are known , R _{EXT can be calculated}

→ R _EXT ≤ 96 Ω

Now that we have the external RC values, the selected ADC can be properly settled. If the calculated R _EXT is too high, C _{EXT can be increased and R EXT} can be recalculated to reduce its value, and vice versa. Figure 8 shows the selected value of C _EXT and the corresponding R _EXT value to simplify the calculation task under the working conditions of this example.

Figure 8. The ADC correctly establishes the corresponding external input RC relationship.

Use the previous steps to find appropriate starting values ​​for R _EXT and C _EXT . Benchmarking and evaluation should be performed, and these values ​​should be optimized as necessary, keeping in mind the impact of such changes on performance.

This article introduces a new family of instrumentation amplifiers that help connect sensors to data acquisition devices. The article explores the characteristics of these devices in detail and uses a practical example to show how to design ADC front-end components to ensure that the combination of driver and ADC can provide the expected resolution.

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