UNLESS OTHERWISE NOTED, this document contains PRODUCTION DATA.
The THS3091 and THS3095 are high-voltage, low-distortion , high-speed, current feedback amplifiers designed to operate over a wide supply range of ± V to ±15 V for applications requiring large, linear output swings such as Arbitrary Waveform Generators.
The THS3095 features a power-down pin that puts the amplifier in low power standby mode, and lowers the quiescent current from 9.5 mA to 500 uA
The THS3095 features a power-down pin (PD) which lowers the quiescent current from 9.5 mA down to 500 μA, ideal for reducing system power.
The power-down pin of the amplifier defaults to the positive supply voltage in the absence of an applied voltage, putting the amplifier in the power-on mode of operation. To turn off the amplifier in an effort to conserve power, the power-down pin can be driven towards the negative rail. The threshold voltages for power on and power down are relative to the supply rails and are given Typical Characteristics (±15 V) and Typical Characteristics (±5 V) tables. Above the Enable Threshold Voltage, the device is on. Below the Disable Threshold Voltage, the device is off. Behavior in between these threshold voltages is not specified.
Note that this power-down functionality is just that; the amplifier consumes less power in power-down mode. The power-down mode is not intended to provide a high-impedance output. In other words, the power-down functionality is not intended to allow use as a 3-state bus driver. When in power-down mode, the impedance looking back into the output of the amplifier is dominated by the feedback and gain-setting resistors, but the output impedance of the device itself varies depending on the voltage applied to the outputs.
Figure 59 shows the total system output impedance which includes the amplifier output impedance in parallel with the feedback plus gain resistors, which cumulate to 2380 Ω. Figure 60 shows this circuit configuration for reference.
As with most current feedback amplifiers, the internal architecture places some limitations on the system when in power-down mode. Most notably is the fact that the amplifier actually turns ON if there is a ±0.7 V or greater difference between the two input nodes (V+ and V–) of the amplifier. If this difference exceeds ±0.7 V, the output of the amplifier creates an output voltage equal to approximately [(V+ – V–) –0.7 V] × Gain. This also implies that if a voltage is applied to the output while in power-down mode, the V– node voltage is equal to VO(applied) × RG/(RF + RG). For low gain configurations and a large applied voltage at the output, the amplifier may actually turn ON due to the aforementioned behavior.
The time delays associated with turning the device on and off are specified as the time it takes for the amplifier to reach either 10% or 90% of the final output voltage. The time delays are in the order of microseconds because the amplifier moves in and out of the linear mode of operation in these transitions.
In addition to the power-down pin, the THS3095 features a reference pin (REF) which allows the user to control the enable or disable power-down voltage levels applied to the PD pin. In most split-supply applications, the reference pin is connected to ground. In either case, the user needs to be aware of voltage-level thresholds that apply to the power-down pin. The tables below show examples and illustrate the relationship between the reference voltage and the power-down thresholds. In the table, the threshold levels are derived by the following equations:
The recommended mode of operation is to tie the REF pin to midrail, thus setting the enable/disable thresholds to Vmidrail + 2 V and Vmidrail + 0.8 V respectively.
Note that if the REF pin is left unterminated, it will float to the positive rail and will fall outside of the recommended operating range given above (VS– ≤ VREF ≤ VS+ – 4 V). As a result, it will no longer serve as a reliable reference for the PD pin and the enable/disable thresholds given above will no longer apply. If the PD pin is also left unterminated, it will also float to the positive rail and the device will be enabled. If balanced, split supplies are used (±Vs) and the REF and PD pins are grounded, the device will be disabled.
The THS309x are unity gain stable 235-MHz current-feedback operational amplifiers, designed to operate from a ±5-V to ±15-V power supply.
Figure 60 shows the THS3091 in a noninverting gain of 2-V/V configuration typically used to generate the performance curves. Most of the curves were characterized using signal sources with 50-Ω source impedance, and with measurement equipment presenting a 50-Ω load impedance.
Current-feedback amplifiers are highly dependent on the feedback resistor RF for maximum performance and stability. Table 4 shows the optimal gain-setting resistors RF and RG at different gains to give maximum bandwidth with minimal peaking in the frequency response. Higher bandwidths can be achieved, at the expense of added peaking in the frequency response, by using even lower values for RF. Conversely, increasing RF decreases the bandwidth, but stability is improved.
|THS3091 and THS3095 RF and RG values for minimal peaking with RL = 100 Ω|
|GAIN (V/V)||SUPPLY VOLTAGE (V)||RG (Ω)||RF (Ω)|
|2||±15||1.21 k||1.21 k|
|±5||1.15 k||1.15 k|
|–1||±15 and ±5||1.05 k||1.05 k|
|–2||±15 and ±5||499||1 k|
|–5||±15 and ±5||182||909|
|–10||±15 and ±5||86.6||866|
The THS309x have the capability to operate from a single-supply voltage ranging from 10 V to 30 V. When operating from a single power supply, biasing the input and output at mid-supply allows for the maximum output voltage swing. The circuits shown in Figure 62 show inverting and noninverting amplifiers configured for single-supply operations.