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芯片采用QFP64封装,如图8(a)所示。将芯片和探测器放置杜瓦内,在低温液氮环境下测试。探测器通过键压的方式与芯片间接互连。探测器的暗电流如图1 (b)所示。电路测试系统如图8(b)所示。采集系统采用NI PXIe-1062Q,其中NI PXI-6552板卡提供时钟信号,NI PXIe-5122板卡采集所需的信号数据。
首先对背景抑制结构进行测试,采用源表Keithley 6430来测试在不同偏压下的电压−电流背景抑制结构的输出特性,如图9(a)、(b)所示。图9(a)为常温下补偿电流的输出曲线,图9(b)为80 K时的补偿电流的输出曲线。常温下补偿电流的输出范围为0~1.1 μA范围内,与仿真结果一致。在0~2 V范围内具有良好线性度,通过拟合其线性度为99.7%。测试温度为80 K时,补偿电流的输出范围为0~2.3 μA。在0~2 V范围内拟合度为99.91%。当背景电流较小即不在补偿电流的线性范围内,通过往小的电流范围进行粗调整,再通过CMBDS模块对其残差的背景电流进行自适应的记忆和抑制。低温下电路的噪声较低且载流子迁移率相对增大,使得其输出范围以及线性度相对于常温下有着较大的提高。
图9(c)、(d)为电流存储型背景抑制的记忆精度测试图,测试温度为80 K。图9(c)为通过Keithley 6430输入电流的方式进行记忆精度的误差分析,以有效信号电流20 nA的积分电压作为参考。记忆20 nA时的背景电流,对40 nA的信号电流进行积分即通过背景抑制有效积分电流为20 nA。当信号电流为60 nA时,其背景电流设置为40 nA,以此类推进行精度测试。从图9(c)可以看出随着记忆的电流增大,背景记忆的精度逐渐提高,与仿真结果一致。差模背景抑制通过背景记忆时信号放大,背景抑制时信号缩小来提高背景抑制精度。当记忆电流大于130 nA时,其记忆的误差小于1%。图9(d)为读出电路与探测器耦合后以温差为15 ℃的响应电压作为参考。记忆温度为20 ℃时的背景电流,对黑体为35 ℃的信号电流进行积分,以此类推。从图9(d)可以看出随着黑体辐射的温度越高,信号电流越大,背景记忆的精度逐渐提高。
基于国标红外焦平面阵列参数测试方法,对长波红外焦平面进行测试。图10为焦平面的测试结果,首先测试了黑体温度为20 ℃时电路输出信号与积分时间的关系,如图10(a)所示。在输出线性范围内线性度大于99.9%。长波红外焦平面功耗为27.36 mW。开启背景抑制后其功耗约为28 mW,基于其功耗的差值可以计算出单元背景抑制功耗约为40 μW。对于长波红外探测器其暗电流水平在几百纳安范围内,因此,共模背景抑制模块的补偿电流可以设置为1 μA范围内。差模背景抑制功耗较大,主要是由于电流镜放大以及缩小模块导致。在大面阵应用中,通过降低对CMBDS的电流镜放大倍数、采用CMBDS共享模式以及限制VIBDS的输出范围来降低其背景抑制模块的总功耗。图10(b)为焦平面所有像元的响应图,像元平均响应率为1.48×107 V/W,积分间为100 μs。未开启背景抑制时,焦平面FPN值为48.25 mV,RMS噪声为0.597 mV。开启背景抑制后,其FPN值下降为5.8 mV,RMS噪声上升为0.681 mV。读出电路未耦合探测器时,其FPN值为2.08 mV,RMS噪声为0.235 mV。图2(d)为所用长波探测器的暗电流非均匀分布,其均方差为1.227 nA。当积分时间为100 μs时,通过该值计算由暗电流非均匀性所产生的FPN理论值约为40.9 mV。该值与未开启背景抑制时的FPN噪声差7 mV左右。电路具体参数指标如表1所示。通过该表可以看出:开启背景抑制功能后,焦平面的FPN值下降,其RMS噪声以及功耗稍微增大。
Performance ROIC FPA (BDS off) FPA (BDS on) Supply voltage/V 5 Dynamic range/dB 78.59 70.5 69.35 RMS(noise)/V 2.35×10−4 5.97×10−4 6.81×10−4 Temperature/K 80 Background suppression range/μA 0~2 Memory current error (>130 nA) <1% FPN/V 0.002 0.0482 0.005 Power/mW 27 27.36 28 Table 1. Performance parameters of test
Long wavelength infrared readout circuit with background suppression function
doi: 10.3788/IRLA20200266
- Received Date: 2020-07-02
- Rev Recd Date: 2020-09-08
- Available Online: 2021-02-07
- Publish Date: 2021-02-07
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Key words:
- background suppression /
- dark current /
- readout circuit /
- LWIR
Abstract: In order to improve the sensitivity of target detection by IRFPA, the carrier generated by target radiation should be maintained as long as possible. And the proportion of thermal excitation and background radiation excitation should be reduced as much as possible. The integral capacitance of the long-wave infrared (LWIR) readout circuit (ROIC) is easily saturated under high background conditions. And the non-uniformity of the LWIR detector dark current will affect the fixed pattern noise (FPN) of the focal plane array (FPA). Based on the common mode background suppression (BDS) structure and the analysis of dark current for long-wave HgCdTe detector, the BDS circuit with non-uniformity correction was designed. Traditional background suppression circuits only used common mode background suppression or differential mode background suppression. The high-precision background memory of the differential mode background suppression module was generally within a small range. Common mode BDS and differential mode BDS were used for BDS module in this paper, which can effectively reduce the fixed graphics noise and increase the dynamic range in a larger background noise range. For this background suppression circuit, the common mode background suppression used a voltage-current conversion method, and the differential mode background suppression used a current storage type background suppression structure. The background signal was amplified during background memory and signal was reduced during BDS for differential mode BDS. It could improve BDS accuracy. The circuit adopted standard CMOS process tape out. The test result shows that the FPN of ROIC is 2.08 mV. The FPN of the FPA without background suppression is 48.25 mV. When background suppression is turned on, its FPN noise is 5.8 mV. Based on the detector's non-uniform distribution of dark current, the theoretical FPN value is calculated to be 40.9 mV. The RMS noise of the output signal of the long-wave infrared focal plane is about 0.6 mV.