An analog-to-digital converter, or A/D converter, or ADC for short, usually refers to an electronic component that converts an analog signal into a digital signal. A typical analog to digital converter is a digital signal that converts an input voltage signal into an output. Since the digital signal itself does not have practical meaning, it only represents a relative size. Therefore, any analog-to-digital converter requires a reference analog quantity as the conversion standard. The more common reference standard is the maximum convertible signal size. The output digital quantity represents the size of the input signal relative to the reference signal.
LTC2387-18 converter circuit diagram
The LTC2387-18 is a 15Msps, highly linear, low noise SAR converter with differential inputs. The ADC's superior linearity and wide dynamic range make it ideal for high-speed imaging and instrumentation applications. Delay-free operation provides a unique solution for high-speed control loop applications. Very low distortion at high input frequencies enables communication applications that require a wide dynamic range and large signal bandwidth.
In most cases, performance is optimized by driving the ADC input with a differential input, differential output amplifier. Where only a single-ended signal is available, a high-performance op amp is required to convert a single-ended signal into a differential signal for the LTC2387-18.
In the above circuit, two op amps are used, one for the in-phase path and the other for the inverting path. The in-phase op amp is a voltage follower with an RC low-pass filter in front of it. This filter prevents very high frequency signals from reaching the LT6201, which can operate at frequencies of tens of MHz. The inverting op amp is driven by an identical network. To invert the signal, R5 and R7 are set to 590Î©. These resistor values â€‹â€‹reflect the trade-off between op amp output current, input offset current, and noise generation. The selected resistor value plus the 4V peak-to-peak output voltage swing results in a 3.3mA peak current drawn from the op amp output. Using a smaller resistor value helps reduce offset and noise, but it draws more current from the op amp and produces more distortion.
There are several tradeoffs for any single-ended to differential conversion circuit. The first is the resulting DC offset; using two separate op amps can contribute to the adverse effects of DC offsets. The main cause of this condition is the inverting op amp portion of the circuit, which must include additional resistors to achieve inversion. The LT6201's input offset current can be as high as 4Î¼A with a 1mV input offset voltage. These can produce a differential DC offset of approximately a few mV at the output of the circuit, which can reach tens of LSBs. To minimize this effect, choose an op amp with low input bias current, offset current, and offset voltage. The second problem is related to the voltage reference Vcm in the circuit shown. Any noise on this node will be directly mapped as differential noise to the inverting terminal of the ADC. Therefore, the voltage source must be as quiet as possible to avoid reducing the SNR of the ADC. For the LTC2387-18, the typical SNR can be as high as 97dBFS. Since the full scale is 8.192Vpk-pk, the noise floor of the ADC reaches approximately 41Î¼VRMS in a 7.5MHz bandwidth. For example, if the noise level of the voltage reference is 41Î¼VRMS, it will reduce the SNR of the ADC by 3dB. Therefore, the noise of the reference must be much lower than this value, so that the SNR of the ADC is not reduced. Note that if the DC common-mode level of the input signal is different from the Vcm in the circuit shown, there will be a differential DC offset in the signal delivered to the ADC.
Therefore, when using this circuit, the low offset level of the op amp and an accurate low noise voltage reference are critical to achieving the full performance of the ADC. The LTC6655 is a very suitable device for Vcm, which is a precision bandgap voltage reference that provides superior noise and drift performance.
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