The LTC2387-18 is a 15Msps, highly linear, low-noise SAR converter with differential inputs. The combination of excellent linearity and wide dynamic range makes this ADC ideal for high speed imaging and instrumentation applications. No-latency operation provides a unique solution for high speed control loop applications. The very low distortion at high input frequencies enables communications applications requiring wide dynamic range and significant signal bandwidth.
In most cases, performance is optimized by using a differential in, differential out amplifier to drive the ADC inputs. In the case where only a single-ended signal is available, high-performance op amps are needed to convert a single-ended signal to a differential signal that is suitable for the LTC2387-18. The LT6201 is a dual ultralow noise, rail-to-rail input and output unity gain stable op amp that feature 0.95nV/√Hz noise voltage. This amplifier combine very low noise with a 165MHz gain bandwidth, 50V/μs slew rate and are optimized for low voltage signal conditioning systems.
In the above circuit, two op amps are used—one for the noninverting path and another for the inverting path. The noninverting op amp is a voltage follower, preceded by a RC lowpass filter. This filter prevents very high frequency signals from reaching the LT6201, which is capable of operation at tens of MHz. The inverting op amp is driven by an identical network. To invert the signal, R5 and R7 are set for 590Ω. These resistor values reflect a compromise between op amp output current, input offset current, and noise contribution. The values selected, coupled with the 4V peak-to-peak output voltage swing, result in a 3.3mA peak current drawn from the op amp output. Smaller resistor values would make for less offset and noise, but would draw more current from the op amp and generate more distortion.
There are a couple of trade-offs with any single-ended to differential conversion circuit. The first one is the resulting DC offset; using two separate op amps can make for increased DC offset effects. The prime reason for this is the inverting op amp portion of the circuit, which must include additional resistors to effect the inversion. The input offset current of the LT6201 may be as high as 4µA, with an input offset voltage of 1mV. These can yield a differential DC offset at the output of the circuit on the order of several mV, which can amount to several tens of LSB’s. To minimize this effect, select an op amp with low input bias current, offset current, and offset voltage. The second issue has to do with the voltage reference Vcm in the circuit shown. Any noise on this node will map directly as differential noise to the inverting terminal of the ADC. Therefore, this voltage source must be as quiet as possible to avoid degrading the SNR of the ADC. In the case of the LTC2387-18, the typical SNR may be as high as 97dBFS. Since full scale is 8.192Vpk–pk, the noise floor of the ADC amounts to about 41µVRMS in a 7.5MHz bandwidth. If the noise level of the voltage reference were, for example, 41µVRMS it would degrade the SNR of the ADC by 3dB. The noise of the reference must therefore be much lower than this so that it will not reduce the SNR of the ADC. Note that if the DC common-mode level of the input signal differs from Vcm in the circuit shown, there will be a differential DC offset in the signal delivered to the ADC. So low levels of offset from the op amp and an accurate low noise voltage reference are critical to realizing the full performance from the ADC when using this circuit. One device that is ideal for Vcm is the LTC6655, a precision bandgap voltage reference that offering exceptional noise and drift performance.