Guy Hoover, Linear Technology
Design Note 526
The LTC2314-14 is a 14-bit, 4.5 Msps, serial output ADC with an integrated high performance reference. The single-ended input of the LTC2314-14 is easy to drive and in many instances does not require a buffer. A driver, such as the LT6236 op amp, may be required for a signal that is small or has high output impedance. The LT6236 is a 215 MHz gain bandwidth product, rail-to-rail output op amp/SAR ADC driver that features 1.1 nV/vHz input-referred noise voltage density and draws only 3.5 mA of supply current with a typical offset of only 100 µV. The LT6236 is a good choice for these applications because of its high bandwidth, low noise, low supply current and low offset.
The driver presented here is characterized over a range of gains, sampling frequencies and input frequencies to establish its application suitability.
|Figure 1.||Single-Ended ADC Driver with Up to 0 V to 4.096 V Input Range.|
Figure 1 shows a non-inverting amplifier driving the LTC2314-14. The driver has a gain between one and ten depending on the value of R3, as shown in Table 1. Capacitor C1 and resistor R1 limit the input bandwidth to 68 MHz. C1 also acts as a charge reservoir for the ADC sample-and-hold capacitor and helps to isolate the LT6236 from the transient that occurs at AIN when the ADC goes into sample mode. R1 is also used to help isolate the op amp from the ADC sampling transient.
|Table 1.||Gain and Input Range
Settings for Various
Values of R3
|Figure 2.||16k-Point FFT for the Circuit of Figure 1.|
The FFT in Figure 2 shows that with a gain of one this circuit has an SNR of 77 dB and a THD of –84 dB, with a sampling rate of 4.5 Msps and a 600 kHz input frequency. These numbers are close to the typical data sheet performance specifications for the LTC2314-14 alone, indicating that there is minimal performance degradation of the ADC when using this driver. Figure 3 shows SINAD performance vs sampling rate for gains of one through ten. The SINAD remains about the same for all gains at approximately 75 dB to 76 dB, with sampling rates from 1 Msps to 4.5 Msps, which is the maximum rated sampling frequency for this part.
|Figure 3.||SINAD vs Sampling Rate.|
Figure 4 shows SINAD performance vs input frequency for gains of one through ten. The SINAD starts at 75 dB to 76 dB for an input frequency of 100 kHz but falls as the input frequency and gain rises. For most applications a drop in SINAD of 3 dB should be acceptable. At a gain of one, SINAD is reduced to 73 dB beyond 2.2 MHz. At a gain of two, SINAD is reduced to 73 dB at approximately 1.2 MHz. At a gain of five, SINAD is reduced to 73 dB at approximately 600 kHz. At a gain of ten, SINAD is reduced to 73 dB at approximately 250 kHz.
|Figure 4.||SINAD vs Input Frequency.|
The LTC2314-14, 4.5 Msps, 14-bit serial sampling ADC can be driven by the LT6236 rail-to-rail output, 215 MHz low noise op amp/SAR ADC driver with gains ranging from one through ten, sample rates from 1 Msps to 4.5 Msps and input frequencies ranging from 100 kHz to 2.2 MHz. This driver circuit is appropriate for small or high output impedance signals. The results shown in Figures 3 and 4 can be used to assess the suitability of this driver for an application with a particular input signal bandwidth and gain.