The variable gain sub femto ampere amplifier DDPCA-300 is the latest addition to FEMTO's broad range of low noise transimpedance amplifiers. Its exceptional design achieves input noise values down to 0.2 fA/√Hz and 0.4 fA peak-to-peak. The gain can be set over a very wide range from 104 up to 1013 V/A either manually or by a remote interface. Thus, the amplifier covers a large dynamic range of more than 240 dB for the measurement of currents from sub femto amps up to milli amps by simply switching the gain range. The compact housing is optimized for use close to the signal source avoiding signal perturbations or noise pick-up due to long cables. Various filters are integrated to adapt the amplifier bandwidth to varying signal conditions. Thus, the signal to noise ratio and measurement speed can be optimized by selecting the proper filter setting for a specific application. And even in the highest gain settings the measurement speed is sufficient to measure femto amp currents in real-time without the need for external averaging or long test periods (see figure 1 below).
At the femto ampere level drift and offset currents can easily cause measurement errors and shifting baselines. The design of the DDPCA-300 is optimized for long-term stability and extremely low drift. The below figure shows the amplifier's baseline drift of less than 0.5 fA measured over a period of more than 7 hours. In case of inevitable (e.g. temperature related) offsets an trimpot is provided for cancelling unwanted offset currents and for nulling of the baseline.
Device characterization by source Biasing
The very high sensitivity of the DDPCA-300 can be utilized in low current applications like characterization of high resistance semiconductors (e.g. MOS or JFETs) or quantum-dot devices. An adjustable voltage is supplied directly at the amplifier input for biasing the device under test (DUT). Changing the bias voltage manually or through the remote interface in the range of ±10 V allows the characterization of the DUT for different bias voltages in spectroscopic measurements. For applications not requiring a bias voltage like photocurrent measurements or beam monitoring in particle physics the bias voltage can be easily disabled.
|Bandwidth* (-3 dB) [Hz]||400||400||400||400||150||150||20||20||1||1|
|Rise Time* (10 % - 90 %) [ms]||0.8||0.8||0.8||0.8||2.3||2.3||17||17||350||350|
|Integrated Input Noise* (Peak-Peak)
||7 nA||7 nA||70 pA||70 pA||1.2 pA||1.2 pA||50 fA||50 fA||2 fA||2 fA|
|Spectral Input Noise Density [/√Hz]||45 pA||45 pA||0.5 pA||0.5 pA||15 fA||15 fA||1.3 fA||1.3 fA||0.2 fA||0.2 fA|
|Accuracy Performance||Gain ±1 %|
|Adjustable Low Pass Filter||3 settings: full bandwidth, 0.7 Hz and 0.1 Hz|
|Output Performance||±10 V, ±30 mA|
|Bias Voltage||±10 V, max. 10 mA, connected to amplifier input, adjustable by trimpot or remote control voltage|
|Power Supply||±15 V, +70 mA / -15 mA typ., ±150 mA recommended|
|Control Interface||4 opto-isolated digital inputs, TTL/CMOS compatible, analog bias control voltage input|
|Case||170 x 60 x 45 mm (L x W x H), Weight 320 g (0.74 lb.)|
|View and Download Datasheet||278 kB|
* The values for bandwidth, rise time and integrated input noise stated in the table above are achieved with the low pass filter set to full bandwidth. Lower peak-to-peak noise values can be achieved by setting the low pass filter to 0.7 Hz or 0.1 Hz. The minimum peak-to-peak noise of 0.4 fA is achieved in the gain settings 1012 and 1013 V/A with the low pass filter set to 0.1 Hz.
Offset adjustable by trimpot. Overload indication by LED and digital control output. Input protected against ±2 kV transients. Output short-circuit protected. Power supply via 3-pin Lemo® socket. A mating connector is provided with the device. Optional power supply PS-15 available. For further information please view the datasheet or contact FEMTO.
Figure 1: Femto Ampere Measurements in Real-Time without Averaging
Figure 2: Long Term Drift
The photoreceiver series HSA-X-S was broadened by two models featuring an improvement in noise performance of up to 40%. The output reflection coefficient was reduced by 8 dB at the same time.
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