Datasheet LTC1414 (Analog Devices) - 10

LTC1414
U U W U APPLICATIONS INFORMATION Choosing an Input Amplifier AC Coupled Inputs
Choosing an input amplifier is easy if a few requirements In applications where only the AC component of the analog are taken into consideration. First, to limit the magnitude input is important, it may be desirable to AC couple the of the voltage spike seen by the amplifier from charging input. This is easily accomplished by DC biasing the the sampling capacitor, choose an amplifier that has a low LTC1414 analog input with a resistor to ground and using output impedance (<100Ω) at the closed-loop bandwidth a coupling capacitor to the input. Figure 7 shows a simple frequency. For example, if an amplifier is used in a gain of AC coupled input circuit for the LTC1414 using only two 1 and has a unity-gain bandwidth of 50MHz, then the additional components. C1 is a 10µF ceramic capacitor output impedance at 50MHz must be less than 100Ω. The and R1 is a 1000Ω resistor to ground. R1 and C1 form a second requirement is that the closed-loop bandwidth highpass filter with a lower cut off frequency of 1/2π(C1)R1 must be greater than 40MHz to ensure adequate small- or 15.9Hz. signal settling for full throughput rate. If slower op amps are used, more settling time can be provided by increasing C1 10µF the time between conversions. 1 A + ANALOG INPUT IN R1 The best choice for an op amp to drive the LTC1414 will 1k 2 A – depend on the application. Generally applications fall into IN LTC1414 two categories: AC applications where dynamic specifica- 3 VREF 1µF tions are most critical and time domain applications where 4 DC accuracy and settling time are most critical. The REFCOMP 10µF following list is a summary of the op amps that are suitable 5 AGND for driving the LTC1414. More detailed information is LTC1414 • F07 available in the Linear Technology Databooks and on the
Figure 7. AC Coupled Input
LinearViewTM CD-ROM.
LT®1223:
100MHz Video Current Feedback Amplifier.
Differential Drive
6mA supply current. ±5V to ±15V supplies. Low noise. Good for AC applications. In some applications the ADC drive circuitry is differential. The differential drive can be applied directly to the LTC1414
LT1227:
140MHz Video Current Feedback Amplifier. 10mA without any special translation circuitry. Differential drive supply current. ±5V to ±15V supplies. Low noise. Best for can be advantageous at high frequencies (>1MHz) since it AC applications. provides improved THD and SFDR. Transformers can be
LT1229/LT1230:
Dual and Quad 100MHz Current Feed- used to provide AC coupling, input scaling and single back Amplifiers. ±2V to ±15V supplies. Low noise. Good ended to differential conversion as shown in Figure 8. The AC specifications, 6mA supply current each amplifier. resistor RS across the secondary will determine the input impedance on the primary. The input impedance of the
LT1360:
50MHz Voltage Feedback Amplifier. 3.8mA sup- primary RP will be related to the secondary load resistor RS ply current. Good AC and DC specs. ±5V to ±15V supplies. by the equation 70ns settling to 0.5LSB. RP = RS/n2
LT1363:
70MHz, 1000V/µs Op Amps. 6.3mA supply cur- rent. Good AC and DC specifications. 60ns settling to For example, if a Minicircuits T4-6T transformer is used, 0.5LSB. the turns ratio is 2; if RS is 200Ω then RP is equal to 50Ω.
LT1364/LT1365:
Dual and Quad 70MHz, 1000V/µs Op The center tap of the secondary will set the common Amps. 6.3mA supply current per amplifier. 60ns settling mode voltage and should be grounded for optimal AC to 0.5LSB. performance. LinearView is a trademark of Linear Technology Corporation. 10